MRS JANDY

Genetics and Inheritance

A Quick Review

A gene is a section of DNA that is transcribed and translated into a single protein

Each chromosome has up to 25,000 genes

Humans have 46 chromosomes. (23 homologous from mom and 23 homologous from dad)

Gregor Mendel

A monk who began studying pea plants in 1843

Discovered and described the basic principles of heredity (how genes are passed from parents to offspring) Mmm…

peas….

Mendel’s Experiments

Mendel started fertilizing pea plants by hand He realized that if he bred a

“tall” plant with another “tall” plant that the offspring would all be tall. Parents called P1 generation Offspring called F1 (fillial)

He called the parents “purebreds” as they produced offspring that looked exactly like them

Studied seed shape, plant height, pod color, flower color…

X

P1

F1

Mendel’s Experiments

He then bred pure plants with different characteristics (green pod plant with yellow pod plant) and the offspring (F1) all turned out green!Where did the yellow pods go?He called these offpsring hybrids

(offspring produced by breeding two different pure lines)

He then bred these F1 plants to produce an F2 generation¼ of the F2 generation plants had yellow

pods and ¾ had green pods. What gives?Clearly he thought something

strange was going on…

F1

F2

X

Mendel’s Hypothesis

Mendel then hypothesized that there are two possibilities for each trait (green or yellow pods) He called the green pods a dominant trait because it was

a more powerful trait that showed up more often He called the yellow pods a recessive trait because it

sometimes disappeared and showed up less oftenHe then realized that if an offspring had one

dominant (green) and one recessive trait (yellow), the dominant trait would show up (green pods)

If the offspring had two recessive traits, the recessive trait would show up (yellow pods)

Mendel’s Hypothesis

He did the same experiment except with plants that had purple and white flowers and saw the same pattern! Purple = dominant White = recessive

Because the dominant (purple) trait always covered up the recessive (white) trait he called this complete dominance

Stop, Pause and Think!

Think about the following…1)What are sections of DNA that contain heredity information ?

2) How does a purebred differ from a hybrid?

3) In a cross that displays completed dominance, if an offspring carries 1 dominant factor and 1 recessive factor, which trait will the offspring have?

Stop, Pause and Think!

1) What are sections of DNA that contain heredity information ?genes2) How does a purebred differ from a hybrid?A purebred the offspring always has the same traits as the parent A hybrid is the result of breeding two different purebreds.3) In a cross that displays completed dominance, if an offspring carries 1 dominant factor and 1 recessive factor, which trait will the offspring have? The dominant trait

Stop, Pause and Think!

You should be able to define the following(Write down in notebook)

1)Gene2)Purebred3)Hybrid4)Complete dominance5)Dominant trait6)Recessive trait

Principal of Segregation

Each chromosome has 2 copies of a gene or trait (one on each chromatid)

These two chromatids separate, or segregate during meiosis when gametes are formed

Each parent contributes one of its copies of a trait to its offspring

The chances of contributing either factor are equal (50/50)

Alleles

We now know that the units of heredity are genes, and the different forms of the genes are called alleles

If the offspring has two dominant alleles, the offspring will appear to show the dominant trait

If the offspring has one dominant allele and one recessive allele, the offspring will show the dominant trait

If the offspring has two recessive alleles, the offspring will show the negative trait

Green + Green =

Green + Yellow =

Yellow + Yellow =

Alleles

Each individual carries one copy of a gene (allele) from their mother and one copy of a gene (allele) from their father

Chromosome #3from mother

Chromosome #3from father

Representing Genes and Alleles

Scientists use abbreviations to show dominant and recessive alleles

They use the same letter for the dominant and recessive allele for each trait Dominant allele is capitalized: Green pods (G) Recessive alleles are lower case: Yellow pods (g)

Genotype vs Phenotype

Genotype: which copies of the gene the organism has What the genes

code for

Phenotype: which trait does the organism show What you see

Determining Genotype

If you know the phenotype can you determine the genotype? If the pea plant has purple flowers: could have one dominant

and one recessive (Pp), or two dominant (PP) alleles If the pea plant has white flowers: must have two recessive

(pp) allelesIf the organism has two of the same allele, the

organism is called homozygous PP = homozygous dominant pp = homozygous recessive

If the organism as one of each allele, the organism is called heterozygous (Pp)

Punnett Squares

A way to visualize test crosses (breeding two organisms)

Can be used to determine the probability of genotypes and phenotypes of offspring

Punnett Squares

Now you try!

? ?

? ?

Stop, Pause and Think!

Think about the following…1) What does the principle of segregation state?

2) What is an allele?

3) What is the difference between a genotype and a phenotype?

4) Define homozygous and heterozygous.

Stop, Pause and Think!

1) What does the principle of segregation state?Members of each pair of genes separate when gametes are formed 2) What is an allele?Different representations of a gene3) What is the difference between a genotype and a phenotype?Genotype is the representation of the alleles; ex: BB or bbPhenotype is the physical representation of the trait; ex: Black or white4) Define homozygous and heterozygous.Homozygous is when both alleles are the same: ex: BB or bbHeterozygous is when you have one dominant and one recessive allele; ex: Bb

Stop, Pause and Think

On your vocabulary sheet define the following1) Allele2) Genotype3) Phenotype4) Homozygous5) Heterozygous• In your notes write down what the Principle

of Segregation States

Incomplete Dominance

In the pea plants Mendel studied, 1 allele was clearly dominant over the otherHowever, this is not always the case!

Some alleles show incomplete dominance (blend of traits instead of one or the other)Ex: red flowers and white flowers make pink flowers

In incomplete dominance the phenotype of a homozygous dominant individual will be different than the phenotype of the heterozygous individualCRCR = CRCW= CWCW =

Demonstrating Incomplete Dominance

Co-dominance

Co-dominance occurs when both alleles are visible in the phenotype (but not mixed like incomplete dominance!)

Ex: This Camellia flower is not pink, instead its petals have red and white parts

Stop, Pause and Think!

How could you tell the difference between an organism and its offspring that show complete dominance and an incomplete dominance?

What is the difference between incomplete dominance and co-dominance?

Stop, Pause and Think!

How could you tell the difference between an organism and its offspring that show complete dominance and an incomplete dominance?

Organisms with complete dominance will only show two variations of a trait. Organisms with incomplete dominance will show three variations of a trait (one mixed)What is the difference between incomplete dominance

and co-dominance?Incomplete dominance is when two traits are mixed (red + white = pink)Co-Dominance is when two traits are both expressed (red + white = part red and part white)

Stop, Pause and Think!

On your vocabulary sheet define the following1) Incomplete Dominance2) Co-Dominance

Multi-allele Systems

Some traits are the result of more than 2 alleles at a locus (location of an allele on a chromosome)

Ex: ABO blood system IA = produces A antigen on blood cell produces B anti-body in blood serum IB = produces B antigen on blood cell produces A anti-body in blood serum i = produces no antigen on blood cell produces both A and B anti-bodies in blood serum

IA and IB are co-dominant i is recessive

The ABO Blood System

Clots when exposed to B-antigenClots when exposed to A-antigenDoes not clot when exposed to antigensClots when exposed to A or B-antigens

Law of Independent Assortment

Mendel showed that dominant traits do not always show up together (don’t always see green pods and purple flowers in the same plant)

Law of Independent Assortment: 2 or more pairs of alleles separate independently during the formation of gametes Ex: equal chances of inheriting blonde hair allele/brown

eyes allele or blonde hair allele/blue eyes alleleTraits are inherited separately from each

other

Sex Linkage

Autosomal traito A gene carried on one of the 22 pairs of non-sex

chromosomesSex-linked trait

A gene carried on one of the pairs of sex-chromosomes If female XX if male XY

If X chromosome codes for the allele, then females will have 2 copies of the allele and males will only have one copy

Y-linked trait Only males (XY) will have a copy of the allele

Very rare in humans

Showing Sex-Linkage

Symbols are written as superscript of the sex chromosome: Xa - X chromosome carrying the recessive allele XA - X chromosome carrying the dominant allele X - No superscript is used for the normal (wild

type) allele

If you see a different ratio of the trait in males and females its probably a sex-linked trait!

Pedigrees

Pedigrees are used to determine mode of inheritance when few individuals, but several generations are involved

Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait

Symbols:

female not affected male not affected

female affected male affected

female carrier male carrier

Pedigree Analysis

Stop, Pause and Think!

On your vocabulary sheet define the following

1) Multi-allele system

Polygenic Inheritance

Most traits are not limited to two possibilities (green or yellow)

Most traits are a continuum (many act together to determine phenotype)Ex: height, skin color

Polygenetic Inheritance: two or more genes act additively on a trait

Stop, Pause and Think!

Think about the following…1) What is an example of a trait that is the result of multi-allele systems?2) Which blood type is homozygous recessive? 3) How would you be able to tell if a disease was a sex linked trait by looking at a pedigree?

Stop, Pause and Think!

1) What is an example of a trait that is the result of multi-allele systems?Blood type2) Which blood type is homozygous recessive? O blood3) How would you be able to tell if a disease was a sex linked trait by looking at a pedigree?If the trait is present in a higher ratio in one sex when compared to the other

Stop, Pause and Think

On your vocabulary sheet define the following1) Multi-allele system2) Law of Independent Assortment3) Autosomal trait4) Sex-linked trait5) Pedigree6) Polygenic Inheritance

Dihybrid Crosses

Dihybrid cross involves the cross of two organisms while looking at two different genes

Can demonstrate independent assortmentCross organism with two homozygous dominant

and two homozygous recessive genotypes YYRR x yyrr can produce:

YYRR and YyRr = yellow round YYrr = yellow wrinkled yyRR and yyRr = green round yyrr = green wrinkled

Produces these genotypes in a 9:3:3:1 ratio

Diagramming a Dihybrid Cross

How to do a Dihybrid Cross

Lets cross two plants one with (R = red flowers) (T = tall) RrTt and RrTt

Draw out a 4 x 4 Punnentt square

How to do a Dihybrid Cross

Crossing RrTt and RrTtStart at the top and fill out the first allele of

the top line (R and r)

RRrr

How to do a Dihybrid Cross

Crossing RrTt and RrTtNow do the same thing with the second

alleles but alternating (T and t)R R r r

TT

tt

How to do a Dihybrid Cross

Crossing RrTt and RrTtNow do the same thing with the second set of

allelesRT Rt rT rt

RT

Rt

rT

rt

How to do a Dihybrid Cross

Crossing RrTt and RrTtStarting at the top fill out the first alleles on the

whole Punnett square

R R r r

R R r r

R R r r

R R r r

RT Rt rT rt

RT

Rt

rT

rt

How to do a Dihybrid Cross

Crossing RrTt and RrTtDo the same thing with the first allele on the

vertical axis

RR RR rR rR

RR RR rR rR

Rr Rr rr rr

Rr Rr rr rr

RT Rt rT rt

RT

Rt

rT

rt

How to do a Dihybrid Cross

Crossing RrTt and RrTtStarting at the top fill out the second allele

on the whole punnet square

RRT RRt rRT rRt

RRT RRt rRT rRt

RrT Rrt rrT rrt

RrT Rrt rrT rrt

RT Rt rT rt

RT

Rt

rT

rt

How to do a Dihybrid Cross

Crossing RrTt and RrTtNow do the same thing with the second allele

on the vertical axis

RRTT RRtT rRTT rRtT

RRTt RRtt rRTt rRtt

RrTT RrtT rrTT rrtT

RrTt Rrtt rrTt rrtt

RT Rt rT rt

RT

Rt

rT

rt

How to do a Dihybrid Cross

Analyze the data!Make a tally of all possible phenotypes

RRTT RRtT rRTT rRtT

RRTt RRtt rRTt rRtt

RrTT RrtT rrTT rrtT

RrTt Rrtt rrTt rrtt

RT Rt rT rt

RT

Rt

rT

rt

Red/Tall – IIII IIII = 9Red/Short- III = 3White/Tall- III = 3White/Short- I = 1

9:3:3:1 Ratio