Human Genetics Mendelian Genetics.

98
Human Genetics Mendelian Genetics

description

Inheritance Parents and offspring often share observable traits. Grandparents and grandchildren may share traits not seen in parents. Why do traits disappear in one generation and reappear in another? Why do we still keep talking about Mendel and his peas?

Transcript of Human Genetics Mendelian Genetics.

Page 1: Human Genetics Mendelian Genetics.

Human Genetics

Mendelian Genetics

Page 2: Human Genetics Mendelian Genetics.

Inheritance Parents and offspring often

share observable traits. Grandparents and

grandchildren may share traits not seen in parents.

Why do traits disappear in one generation and reappear in another?

Why do we still keep talking about Mendel and his peas?

Page 3: Human Genetics Mendelian Genetics.

Darwin Prior to MendelScientists looked for rules to explain continuous

variation.The “blending” hypothesis: genetic material from

the two parents blends together Head size, height, longevity – all continuous

variations - support the blending hypothesisThe “particulate” hypothesis: parents pass on

discrete heritable units (genes)Mendel’s experiments suggested that inherited

traits were discrete and constant

Page 4: Human Genetics Mendelian Genetics.

Gregor Mendel

Why did Mendel succeed in seeing something that nobody else saw?

1. He counted2. Chose a good system3. Chose true-breeding characters

http://www.mendel-museum.org/eng/1online/experiment.htm

Page 5: Human Genetics Mendelian Genetics.

The field of genetics started with a single paper!

Page 6: Human Genetics Mendelian Genetics.

Mendel is as important as Darwin in 19th century science

Mendel did experiments and analyzed the results mathematically. His research required him to identify variables, isolate their effects, measure these variables painstakingly and then subject the data to mathematical analysis.

He was influenced by his study of physics and having an interest in meteorology. His mathematical and statistical approach was also favored by plant breeders at the time.

Page 7: Human Genetics Mendelian Genetics.

Mendel used an Experimental, Quantitative Approach

Advantages of pea plants for genetic study: There are many varieties with distinct heritable

features, or characters (such as color); character variations are called traits

Mating of plants can be controlled Each pea plant has sperm-producing organs

(stamens) and egg-producing organs (carpels) Cross-pollination (fertilization between different

plants) can be achieved by dusting one plant with pollen from another

Page 8: Human Genetics Mendelian Genetics.

1. Self-fertilization2. Cross-pollination

Page 9: Human Genetics Mendelian Genetics.

Mendel chose to track only those characters that varied in an “either-or” manner

He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate)

He spent 2 years getting “true” breeding plants to studyAt least three of his traits were available in seed catalogs of the

day

Mendel Planned Experiments Carefully

Page 10: Human Genetics Mendelian Genetics.

Mendel studied true breeding pea traits with two distinct forms

Page 11: Human Genetics Mendelian Genetics.

Terminology of BreedingP1 (parental) - pure breeding strain

F1 (filial) – offspring from a parental cross

They are also referred to as hybrids – because they are the offspring of two 2 pure-breeding parents

F2 - produced by self-fertilizing the F1 plants

Page 12: Human Genetics Mendelian Genetics.

LE 14-6Phenotype

Purple

Purple3

Purple

Genotype

PP(homozygous

Pp(heterozygous

Pp(heterozygous

pp(homozygous 1

2

1

Ratio 1:2:1

White

Ratio 3:1

1

Page 13: Human Genetics Mendelian Genetics.

Appearance:

P Generation

Genetic makeup:

Gametes

F1 Generation

Appearance:Genetic makeup:

Gametes:

F2 Generation

Purple flowersPp

P p1 21 2

P pF1 sperm

F1 eggs PP Pp

Pp pp

P

p

3 : 1

Purpleflowers

PP

Whiteflowers

pp

P p

Page 14: Human Genetics Mendelian Genetics.

The TestcrossHow can we tell the genotype of an individual

with the dominant phenotype?This individual must have one dominant allele,

but could be either homozygous dominant or heterozygous

The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual

If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Page 15: Human Genetics Mendelian Genetics.

LE 14-7

Dominant phenotype,unknown genotype:

PP or Pp?

If PP,then all offspring

purple:

p p

P

P

Pp Pp

Pp Pp

If Pp,then 1

2 offspring purpleand 1

2 offspring white:

p p

P

Ppp pp

Pp Pp

Recessive phenotype,known genotype:

pp

Page 16: Human Genetics Mendelian Genetics.

Mendel’s Second Law: The Law of Independent Assortment

Mendel derived the law of segregation by following a single character

The F1 offspring produced in this cross were all heterozygous for that one character

A cross between such heterozygotes is called a monohybrid cross

Page 17: Human Genetics Mendelian Genetics.

Mendel identified his second law of inheritance by following two characters at the same time

Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters

A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

Page 18: Human Genetics Mendelian Genetics.

LE 14-8

P Generation

F1 Generation

YYRR

Gametes YR yr

yyrr

YyRrHypothesis ofdependentassortment

Hypothesis of independent assortment

SpermEggs

YR

Yr

yrYR

YR

yr

Eggs

YYRR YyRr

YyRr yyrr yR

yrPhenotypic ratio 3:1

F2 Generation(predictedoffspring)

YYRR YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

Phenotypic ratio 9:3:3:1

YR Yr yR yr

Sperm

1 21 4

1 4

1 4

1 4

1 43 4

1 2

1 2

1 2

1 4

9 16 3 16 3 16 3 16

1 4 1 4 1 4

Page 19: Human Genetics Mendelian Genetics.

The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation

Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes

Genes located near each other on the same chromosome tend to be inherited together

Page 20: Human Genetics Mendelian Genetics.

ProbabilityRanges from 0 to 1Probabilities of all possible events must add up

to 1Rule of multiplication: The probability that

independent events will occur simultaneously is the product of their individual probabilities.

Rule of addition: The probability of an event that can occur in two or more independent ways is the sum of the different ways.

Page 21: Human Genetics Mendelian Genetics.

Multiplication and Addition Rules Applied to Monohybrid Crosses

The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

Probability in an F1 monohybrid cross can be determined using the multiplication rule

Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

Page 22: Human Genetics Mendelian Genetics.

½ chance of P and ½ chance of p allele results in ¼ chance of each homozygous genotype.

There are two ways to get the heterozygous genotype so it is ¼ + ¼ = ½ Three genotypes give the same phenotype.

Page 23: Human Genetics Mendelian Genetics.

Solving Complex Genetics Problems with the Rules of Probability

We can apply the rules of multiplication and addition to predict the outcome of crosses involving multiple characters

A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously

In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

Page 24: Human Genetics Mendelian Genetics.

YYRR yyrr

YyRr

Male gametes

Female Gametes

YyRr

YyRr

¼

¼

¼

¼

¼ ¼ ¼ ¼ YR Yr yR yr

YR

Yr

yR

yr

YYRR YYRr

YYrR YYrr

YyRR YyRr

YyRr Yyrr

YyRR YyRr

YyRr Yyrr yyRr

yyRr

yyrr

yyRR9/16

3/16

3/16

1/16

Page 25: Human Genetics Mendelian Genetics.

For a dihybrid cross – the chance that 2 independent events occur together is the product of their chances of occurring separately.

The chance of yellow (YY or Yy) seeds= 3/4 (the dominant trait)

The chance of round (RR or Rr) seeds = 3/4 (the dominant trait)

The chance of green (yy) seeds= 1/4 (the recessive trait) The chance of wrinkled (rr) seeds= 1/4 (the recessive trait) Therefore:The chance of yellow and round= 3/4 x 3/4 = 9/16The chance of yellow and wrinkled= 3/4 x 1/4 = 3/16The chance of green and round= 1/4 x 3/4 = 3/16The chance of green and wrinkled= 1/4 x 1/4 = 1/16

Page 26: Human Genetics Mendelian Genetics.

Inheritance patterns are often more complex than predicted by Mendel

The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

Many heritable characters are not determined by only one gene with two alleles

However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Page 27: Human Genetics Mendelian Genetics.

Extending Mendelian Genetics for a Single Gene

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or

recessive When a gene produces multiple phenotypes When a gene has more than two alleles The forensic characteristics usually have more

than two alleles

Page 28: Human Genetics Mendelian Genetics.

The Spectrum of Dominance Complete dominance occurs when phenotypes of

the heterozygote and dominant homozygote are identical

In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Forensic Traits are codominant

Page 29: Human Genetics Mendelian Genetics.

RedCRCR

Gametes

P Generation

CR CW

WhiteCWCW

PinkCRCW

CRGametes CW

F1 Generation

F2 Generation EggsCR CW

CR

CRCR CRCW

CRCW CWCWCW

Sperm

12

12

12

12

12

12

Page 30: Human Genetics Mendelian Genetics.

The Relation Between Dominance and Phenotype

A dominant allele does not subdue a recessive allele; alleles don’t interact

Alleles are simply variations in a gene’s nucleotide sequence

For any gene, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

If you look directly at DNA, you can always detect codominance.

Page 31: Human Genetics Mendelian Genetics.

Frequency of Dominant Alleles

Dominant alleles are not always more common in populations than recessive alleles

For example, one baby out of 400 in the USA is born with extra fingers or toes

The allele for this trait is dominant to the allele for the more common trait of five digits per appendage

In this example, the recessive allele is far more prevalent than the dominant allele in the population

Page 32: Human Genetics Mendelian Genetics.

Multiple AllelesMost genes exist in populations in more

than two allelic formsFor example, the four phenotypes of the

ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

Page 33: Human Genetics Mendelian Genetics.
Page 34: Human Genetics Mendelian Genetics.

Polygenic Inheritance• Quantitative characters are those that vary

in the population along a continuum• Quantitative variation usually indicates

polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance

Page 35: Human Genetics Mendelian Genetics.

LE 14-12

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

AaBbCcAaBbCc

20/64

15/64

6/64

1/64

Frac

tion

of p

roge

ny

Page 36: Human Genetics Mendelian Genetics.

Nature and Nurture: The Environmental Impact on Phenotype

Page 37: Human Genetics Mendelian Genetics.

Relating Mendel’s Laws to Cells• Law of Segregation• Pairs of

characteristics (alleles) separate during gamete formation

• Each cell has two sets of chromosomes that are divided to one set per gamete.

Law of Independent Assortment

The inheritance of an allele of one gene does not influence the allele inherited at a second gene.

Genes on different chromosomes segregate their alleles independently.

Page 38: Human Genetics Mendelian Genetics.

Offspring acquire genes from parents by inheriting chromosomes

Children do not inherit particular physical traits from their parents

It is genes that are actually inheritedGenes are carried on chromosomes.Mendel identified 7 sets of characters- One per

each of the 7 chromosomes in peas, so his law worked out perfectly.

Two characters on the same chromosome are linked together and would have messed up his law.

Page 39: Human Genetics Mendelian Genetics.

Inheritance of GenesGenes are the units of heredityGenes are segments of DNAEach gene has a specific locus on a

certain chromosomeOne set of chromosomes is inherited

from each parentReproductive cells called gametes

(sperm and eggs) unite, passing genes to the next generation

Page 40: Human Genetics Mendelian Genetics.

Sexual ReproductionTwo parents give rise to offspring that have

unique combinations of genes inherited from the two parents.

All humans arise from the joining of 1 egg and 1 sperm cell

100% of a person’s DNA is the same within and throughout a human being’s body.

Whether you look at the cells of a person’s blood, skin, semen, saliva or hair, the DNA and genes will be the same.

Page 41: Human Genetics Mendelian Genetics.

Chromosomes Come in Sets

• Each human cell (except gametes) has 46 chromosomes arranged in pairs in its nucleus

The two chromosomes in each pair are called homologous chromosomes

One of each pair came from your mother and the other came from your father.

Both chromosomes in a pair carry genes controlling the same inherited characteristics

Page 42: Human Genetics Mendelian Genetics.

The sex chromosomes are called X and YHuman females have a homologous pair

of X chromosomes (XX)Human males have one X and one Y

chromosomeThe 22 pairs of chromosomes that do not

determine sex are called autosomes

Page 43: Human Genetics Mendelian Genetics.

Each pair of homologous chromosomes includes one chromosome from each parent

The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father

The number of chromosomes in a single set is represented by n

A cell with two sets is called diploid (2n)For humans, the diploid number is 46 (2n = 46)

Page 44: Human Genetics Mendelian Genetics.

Meiosis reduces of chromosome number from diploid to haploid

The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

Page 45: Human Genetics Mendelian Genetics.

Meiosis is preceded by the replication of chromosomes

Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

The two cell divisions result in four daughter cells

Each daughter cell has only half as many chromosomes as the parent cell

Page 46: Human Genetics Mendelian Genetics.

Key

Maternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1 Possibility 2

Combination 2Combination 1 Combination 3 Combination 4

Daughtercells

Metaphase II

Two equally probablearrangements ofchromosomes at

metaphase I

Page 47: Human Genetics Mendelian Genetics.

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv

Maternal set ofchromosomes (n = 3)

2n = 6Paternal set ofchromosomes (n = 3)

Two sister chromatidsof one replicatedchromosomes

Two nonsister chromatids in a homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

8 Gamete Combinations

Page 48: Human Genetics Mendelian Genetics.

LE 13-5

Key

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

Spermcell (n)

TestisOvary

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

FERTILIZATIONMEIOSIS

Diploidzygote(2n = 46)

Page 49: Human Genetics Mendelian Genetics.

Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Page 50: Human Genetics Mendelian Genetics.

Random Fertilization

Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations

Crossing over adds even more variationEach zygote has a unique genetic identity

Page 51: Human Genetics Mendelian Genetics.

LE 13-11Prophase Iof meiosis

Tetrad

Nonsisterchromatids

Chiasma,site of crossingover

Recombinantchromosomes

Metaphase I

Metaphase II

Daughtercells

Page 52: Human Genetics Mendelian Genetics.

  

Monohybrid Cross: - cross involving only one character.

Page 53: Human Genetics Mendelian Genetics.

Results from CrossesF1 offspring -

the trait expressed was the same as that of one of the parental lines

traits did not blendF2 offspring

the traits from both parents were expressed in a 3:1 ratio

while the trait had not been expressed in the F1, it remained unchanged as it was passed from the P1 to the F1 and then to the F2 generation. 

CONCLUSIONTraits are inherited as discrete, separate

units.

Page 54: Human Genetics Mendelian Genetics.

Mendel’s Conclusions

1. Factors (genes) that determine traits can be hidden or unexpressed.

Dominant traits have a factor (gene) that is expressed in the F1 offspring

Recessive traits have a factor (gene) that is not expressed in the F1 offspring

Page 55: Human Genetics Mendelian Genetics.

Mendel’s Conclusions

2. Despite P1 and F1 generations appearing identical, they must be genetically different.

Phenotype- observed properties of a trait

Genotype- the genetic makeup of a trait

PP1 and F1 seeds have the same phenotype but different genotypes

Page 56: Human Genetics Mendelian Genetics.

3. Since the F1 offspring had factors (genes) for both smooth and wrinkled - then there must be at least 2 factors for every trait.

• Alleles- alternative forms of a gene• Genotype- indicates the combination of alleles

present • Phenotype- indicates the trait observed

These terms differentiate the observed form and the underlying alleles present at a particular gene.

Mendel’s Conclusions

Page 57: Human Genetics Mendelian Genetics.

Mendel’s Model

Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

Four related concepts make up this modelThese concepts can be related to what we

now know about genes and chromosomes

Page 58: Human Genetics Mendelian Genetics.

Alternative versions of genes account for variations in inherited characters

For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers

These alternative versions of a gene are now called alleles

Each gene resides at a specific locus on a specific chromosome

The First Concept

Page 59: Human Genetics Mendelian Genetics.

Allele for purple color

Homologouspair ofchromosomes

Allele for white color

Locus for flower color gene

Page 60: Human Genetics Mendelian Genetics.

For each character, an organism inherits two alleles, one from each parent

Mendel made this deduction without knowing about the role of chromosomes

The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation

Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

The Second Concept

Page 61: Human Genetics Mendelian Genetics.

The Third Concept

If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

Page 62: Human Genetics Mendelian Genetics.

The Fourth ConceptKnown as “the law of segregation”Two alleles for a heritable character separate

(segregate) during gamete formation and end up in different gametes

Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism

This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

Page 63: Human Genetics Mendelian Genetics.

Mendel’s Laws Explain his DataMendel’s segregation model accounts for the 3:1

ratio he observed in the F2 generation of his numerous crosses

The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

Page 64: Human Genetics Mendelian Genetics.

It is the basis for Mendel’s Law of Segregation

Pairs of characteristics separate during gamete formation.

What does the 3:1 ratio of dominant to recessive traits in the F2 population mean?

Page 65: Human Genetics Mendelian Genetics.

Review of meiosis

Page 66: Human Genetics Mendelian Genetics.
Page 67: Human Genetics Mendelian Genetics.

  

Monohybrid Cross: - cross involving only one character.

Page 68: Human Genetics Mendelian Genetics.

Two types of “states”

Homozygous: an individual or a locus carries identical

alleles of a given gene.

Heterozygous: an individual or a locus carries different

alleles of a given gene

Page 69: Human Genetics Mendelian Genetics.

Gametes join randomly to make new individuals

Reginald C. PunnettThe Punnett Square

Page 70: Human Genetics Mendelian Genetics.
Page 71: Human Genetics Mendelian Genetics.

Mendel’s Law of Segregation

Members of a gene pairs (alleles) separate from each other during gamete formation.

The underlying mechanism is separation and then segregation of homologous chromosomes during meiosis.

Key terms: dominant and recessive traits genotype versus phenotype

Page 72: Human Genetics Mendelian Genetics.

Mendel’s Law of Segregation holds true for any cross

Page 73: Human Genetics Mendelian Genetics.

Molecular Basis of Mendel’s factors

Mendel’s factors are called alleles. versions of the same gene or DNA

sequence. differ in DNA sequence at one or more sites.

Page 74: Human Genetics Mendelian Genetics.

Phenotype Genotype Abbreviation of Genotype

Tall plant Homozygous dominant “tall associated” alleles.

TT

Tall Plant Heterozygous Tt

Short Plant Homozygous dominant “short associated” alleles.

tt

Genotype and Phenotype

Page 75: Human Genetics Mendelian Genetics.

Dihybrid CrossesParental lines:

smooth yellow seed wrinkled green seeds

Mendel learned from previous monohybrid crosses smooth was dominant over wrinkled yellow was dominant over green

Pure-breeding smooth yellow genotype SSYY Pure-breeding green wrinkled genotype ssyy

Page 76: Human Genetics Mendelian Genetics.

Dihybrid Cross

?

Page 77: Human Genetics Mendelian Genetics.

 315 smooth and yellow parental108 smooth and green new combo101 wrinkled and yellow new combo32 wrinkled and green parental

9:3:3:1

Dihybrid cross - F2 generation

Page 78: Human Genetics Mendelian Genetics.

Fig 3.8

Dihybrid Cross

Page 79: Human Genetics Mendelian Genetics.

Total smooth 315 + 108 = 423Total wrinkled 101 +32 = 133 ~ 3:1 Total yellow 315 + 101 = 416Total green 108 + 32 = 140 ~ 3:1

Each trait is behaving as expected for a monohybrid cross.

Inheritance of each trait appears to be independent of the other trait.

How does this dihybrid F2 phenotypic ratio of 9:3:3:1 relate to the monohybrid cross

phenotypic ratio of 3:1?

Page 80: Human Genetics Mendelian Genetics.

Mendel’s Conclusion

He reasoned that alleles from one gene pair segregate into gametes independently of the alleles belonging to other traits, producing gametes with all possible combinations of alleles.

Page 81: Human Genetics Mendelian Genetics.

Law of Independent Assortment

The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.

Two genes on different chromosomes segregate their alleles independently.

Page 82: Human Genetics Mendelian Genetics.

Law of independent assortment

Page 83: Human Genetics Mendelian Genetics.

Probability: The likelihood that an event will occur

•No chance of event probability = 0(e.g. chance of rolling 8 on a six-sided die)

•Event always occurs probability = 1 (chance of rolling 1,2,3,4,5,or 6 on a six-sided die)

# on die probability

1 1/6

2 1/6

3 1/6

4 1/6

5 1/6

6 1/6

The probability of an event

= # of chance of event total possible events

The probabilities of all the possible events add up to 1.

Page 84: Human Genetics Mendelian Genetics.

Independent Events• The probability of independent events is

calculated by multiplying the probability of each event.

In two rolls of a die, the chance of rolling the number 3 twice:

Probability of rolling 3 with the first die = 1/6

Probability of rolling 3 with the second die = 1/6

Probability of rolling 3 twice = 1/6 x 1/6 or 1/36

Page 85: Human Genetics Mendelian Genetics.

Independent events

What is the chance of an offspring having the homozygous recessive genotype when both parents are doubly heterozygous?

Page 86: Human Genetics Mendelian Genetics.

Independent Events

Page 87: Human Genetics Mendelian Genetics.

Dependent EventsThe probability of dependent events is calculated

by adding the probability of each event.

In one roll of a die, what is the probability of rolling either the number 5 or an even number?

Probability of rolling 5 or an even number = 1/6 + 3/6 or 4/6

Probability of rolling the number 5 = 1/6

Probability of rolling an even number = 3/6

Page 88: Human Genetics Mendelian Genetics.

Parents are heterozygous for a trait, R.

What is the chance that their child carries at least one dominant R allele?

Probability of child carrying RR = 1/4Probability of child carrying Rr = 1/2

Probability of child carrying R = 1/4 + 1/2 = 3/4

Dependent Events

Page 89: Human Genetics Mendelian Genetics.

1/2

1/2

1/2 1/2

So, Rr genotype = (1/2 x 1/2) x 2 = 1/2 RR genotype is (1/2 x 1/2) = ¼Add these to get the combined probability.

R rR

r

RR

Rr

Rr

rr

Can use to solve more complicated problems:

AaBBccDdEeFFGghhIiJJKk x aaBbCCDdEEffggHhIIjjKk

Page 90: Human Genetics Mendelian Genetics.

Crossing Double Heterozygotes

Page 91: Human Genetics Mendelian Genetics.

Fig 3.10b

Page 92: Human Genetics Mendelian Genetics.
Page 93: Human Genetics Mendelian Genetics.

Fig 3.11

Page 94: Human Genetics Mendelian Genetics.

Dihybrid Cross

From the 16 possible fertilization events

- 9 genotypes - 4 phenotypes 9:3:3:1

Page 95: Human Genetics Mendelian Genetics.

Dihybrid cross

Tetrahybrid cross!

Page 96: Human Genetics Mendelian Genetics.

Statistical Analysis

Simple cross: purple x whiteF1: all purpleF2: 2850 purple, 1150 white

Use Χ2 test to determine likelihood of getting this result by chanceΧ2 = total of (observed-expected)2/expected over all classes"Expected" is from null hypothesis - data fit a 3:1 ratio(2850-3000)2/3000 + (1150-1000)2/1000 = 30!

10x as many, butsame ratio

P is <<less than 5%, so data are significantly different from null hypothesis

Page 97: Human Genetics Mendelian Genetics.

Pedigree Analysis

Page 98: Human Genetics Mendelian Genetics.

Inborn errors of metabolismHint: much more common in first cousin marriages

Garrod, 1902 - human traits followed Mendelian rules