GENETICS

Genetics:

• The study of how traits encoded in our DNA are passed on.

• Used to predict the possible outcomes of a genetic cross–Depends on probability

–We might expect 2/4 offspring to look a certain way, and end up with 0/4 looking that way!

History of Genetics as a science:

Gregor Mendel is the “Father of Genetics” - Austrian monk who studied pea plants to figure out how parents pass traits to their offspring

- Started with parents that were True breeding - means that when they self fertilize, their

offspring all look like them.

Parents: (both true breeding)

white x purple

Expect???

What he got:

So… he crossed two of them….

Expect???

What he got:

These crosses showed that there were “factors” being passed from parent to offspring even if it wasn’t being “used”

Now we call these factors GENES

Genes – control a heritable feature;

Example: Hair color, seed shape, height;

Allele – controls the variation of a feature.Example: brown, blonde, black hair

Characteristic/Gene?

Trait/Allele?

CHARACTERS AND VARIANTS OF EACH

TRAITS

?

TRAITS

?

RARE DOMINANT PHENOTYPE

A chromosome = folded up string of many genes

What are alleles?

Variations of a gene that occupy the same locus on homologous

chromosomes

Locus = position on a chromosome.

P

pT

t

LONG

SHORT

GENE = FLOWER COLORGENE = STEM LENGTH

Terms• Diploid (2n)• HAPLOID (n)• Egg• Sperm• Parent• Meiosis• Testes

• Gamete

• Zygote

• Progeny

• Offspring

• Fertilization

• Ovary

Mendel’s laws of genetics

1. Law of segregation: only one allele for each gene is passed from a parent to the offspring.

Why? Has to do with separation of homologous chromosomes during meiosis.

Segregation of Alleles

Tongue Rolling

2. Law of independent assortment:

Alleles for different genes are passed to offspring independently of each other.

The result is that new combinations of genes present in neither parent is possible.

3. Law of complete dominance – some alleles over power others. So even if both alleles are present, we only “see” the dominant one.

- the “hidden” allele is called recessive

This only applies to SOME genes, not all.

Remember Mendel’s pea plants?

- Purple was crossed with white and we got ALL purple. So which allele is dominant?

Genotype: the alleles that an organism has.

- alleles are abbreviated using the first letter of the dominant trait. (with some exceptions that we will get to)

- a capital letter represents the dominant

ex: P for purple flower allele

- a lower case represents the recessive.

ex: p for white flower allele

All diploid organisms have two alleles for each trait:

- you can have two of the same alleles

Ex: PP or pp

- such an individual is described as Pure or Homozygous.

OR

All diploid organisms have two alleles for each trait:

- you can have two different alleles

Ex: Pp

- such an individual is described as hybrid or heterozygous

Phenotype: physical appearanceExamples: brown hair, widows peak

- the trait that “wins” in the case of complete dominance;

- depends on the combination of alleles

P generation: “parents;” First generation in the cross

F generations: results of the cross;

- F1 – 1st generation; offspring of P generation

- F2 – 2nd generation; offspring of F1 generation

Terminology for Genetic Crosses

Monohybrid cross: cross that focuses on the alleles of a single trait;

How do we show the possibilities?

- punnett square

PUNNETT SQUARE

Allele in Egg 1

Zygote formed if sperm 1

fertilizes egg 1

Allele in Egg 2

Allele in sperm 1

Allele in sperm 2

Zygote formed if sperm 2

fertilizes egg 1

Zygote formed if sperm 1

fertilizes egg 2

Zygote formed if sperm 2

fertilizes egg 2

In pea plants tallness is dominant to short or dwarf. Cross a pure tall male to a pure dwarf female pea plant. Show both ratios phenotype & genotype for the offspring. Now cross two of the F1.

• Take it step by step until you “get it”• Step 1: what are the parent’s genotypes?

–Mom?–Dad?

tt

TT

• Step 2: Set up Punnett Square

t t

T

T

Tt Tt

TtTt

• Step 3: ANSWER THE QUESTION

t t

T

T

Tt Tt

TtTt

Offspring genotypes:

Offspring phenotypes:

• Step 4: Part II

T t

T

t

TT Tt

t tTt

Offspring genotypes:

Offspring phenotypes:

Inheritance Patterns:

Every gene demonstrates a distinct phenotype when both alleles are combined (the heterozygote)

Complete dominance is one - when both alleles are present, only the dominant trait is seen.

Inheritance Patterns:

Incomplete dominance - when both alleles are present, the two traits blend together and create an intermediate trait

INCOMPLETE DOMINANCE

Inheritance Patterns:

Co-dominance - when both alleles are present, both traits are visible

Different notation: Use first letter of the feature with a superscript for the trait. Example: CW or CB for white coat or black coat;

Inheritance Patterns:

Co-dominance - when both alleles are present, both traits are visible

Inheritance Patterns:

Each gene has a specific inheritance pattern. - you will either be told or be given a hint; look at the heterozygote!

Women have two X’s but men only have one.

How do we deal with the genes on the X chromosome?

Sex-linked traitAlleles for the trait are located on the X chromosome in humans.

- works the same in women as all the other traits.

BUT –

- men only inherit one such allele.

Sex-linked traitFor females: have to figure out phenotype based on inheritance pattern.

For Males: phenotype is that of whatever allele they inherit.

Example: color blindness

Seeing color (XC) is dominant to being color blind (Xc)

Identify the sex and trait of the following:

XCY XCXc XCXC

XcXc XcY

Example: Color Blindness

Set up a punnett square crossing a heterozygous normal female with a normal male:

- what is mom’s genotype?

- what is dad’s genotype?

- what gametes can each give?

- what are the offspring’s geno’s?

XC

Cross Number 1:

Xc

XC

Y

XCXC XCXc

Xc YXC Y

What % chance of having color blind daughter?

Son?

SEX-LINKED TRAITS

COLOR BLINDNESS

AFFLICTS 8% MALES AND 0.04% FEMALES.

If we are dominant, how can we figure out our genotype?

What are the possibilities?

Test cross: a cross that determines genotype of dominant parent

- Cross unknown dominant parent (possibilities BB or Bb)

with a recessive parent

then analyze the offspring.

B ?

b

b

Bb ?b

?bBb

If some of the offspring have the recessive trait, then the unknown parent has to be heterozygous

B ?

b

b

Bb ?b

?bBb

If all offspring are dominant, unknown parent HAS to be

homozygous

Multiple alleles: Some genes have more than two variations that exist, although we still only inherit 2

Example: Human blood types

Three alleles:

IA

IB

i

Genotype Phenotype

IAIA A

IAi A

IBIB B

IBi B

IAIB AB

ii 0

Polygenic –

Multiple genes each with 2 alleles

Creates additive/

quantitative effect

SKIN PIGMENTATION

Dihybrid cross:

A cross that focuses on possibilities of inheriting two traits

- two genes, 4 alleles

Black fur is dominant to brown fur

Short fur is dominant to long fur

What is the genotype of a guinea pig that is heterozygous for both black and short fur?

Dihybrid cross:

Parent phenotypes: BbSs x BbSs

Figure out the possible gametes:

Then set up punnett square

Dihybrid cross:

BS Bs bS bs

BS

Bs

bS

bs

Linked Genes: genes that are on the same chromosome.

Does the law of independent assortment apply?

Can they be separated? Will they always separate?

What does this mean? • It means that you can pass on an allele that

you got from your mom and an allele you got from your dad ON THE SAME CHROMOSOME

• However, it is more likely that two alleles that start on the same chromosome will get passed on together.