The Chromosomal Basis of Inheritance

Connecting Mendel's Work to Chromosomes

Thomas Hunt Morgan Studied fruit flies,

(only 4 chromosomes)

Examined the pass age of traits in fruit flies

First person to propose the idea of genes being on chromosomes

Wild Type

The “normal” phenotype for a character is called the wild type Red eyes in fruit

flies The other allele is

called the mutant phenotype White eyes

Mutant!

Wild Type

Crossing Red and White Eyes

When Morgan mated a white-eyed male and a red-eyed female, all offspring were red-eyed

When the F1 generation was mated, half of males had white-eyes, none of the females did

Explanation• Morgan reasoned that the gene for

eye color must be located on the X chromosome

• If males receive the recessive allele from their mother, they express it

• Females have 2 copies of the chromosome, so they must receive a recessive allele from both parents

Original Mating

Red eyed allele is dominant, anyone with a red eyed allele has red eyes

F1 Mating

½ of males are white-eyed

They only have the white eyed allele

Summary

• Genes are located on chromosomes

• Males only have 1 X chromosome and will express whatever allele is on that chromosome

• So males are more likely to show recessive traits on the X chromosome

Stupid (in my humble opinion) naming convention

No capital letters, nothing denoting dominance

The alleles are named based on the mutant gene, i.e. white eyes are w

The wild-type (normal trait allele) is w+

So red eyes are w+

Genes on the Same Chromosome

Let b+ code for grey (normal) and b code for black, the mutant color

vg+ = normal wings

vg= vestigial (mutant wings)

Crossing b+bvg+vg male with bbvgvg female

Independent assortment would predict:

1/4th of offspring are b+bvg+vg

1/4th are bbvg+vg,

1/4th are b+bvgvg

1/4th are bbvgvg

Grey – Normal Wings

Black – Normal Wings

Black - Vestigial

Grey – Vestigial

But...

Most of the offspring were either grey and normal or black and vestigial

Most received b+vg+ or bvg from their father

The genes were inherited together most of the time

Some genes are inherited together

Each chromosome has hundreds or thousands of genes

These genes are more likely to be inherited together

Called linked genes

Usually inherited together

Genetic Recombination Recombinants

have new genotypes

Frequency of recombination is the % of offspring that have new genotypes

0% means genes are inherited together

50% means completely separately

Unlinked Genes

•If you inherit allele A…

•The odds are 50% you will also inherit allele B

Recombination Still Occurs In Linked Genes

During meiosis pieces of the homologous chromosomes switch

This is called crossing over

Increases genetic diversity So recombinant frequency

is never really 0

Geneticists Use Recombinants to Map out Chromosomes

The lower the recombinant frequency, the closer the genes are located on the chromosome

Linked Genes

•If you inherit allele A you are moderately likely to inherit B or C as well

•If you inherit B you are very likely to inherit C as well

•Very unlikely crossing over occurs between B and C

Based on the Recombinant Frequencies, Determine the

order of genesAB- 35% AC- 25% AD-5% BC-10%

BD-30% CD-20%

Create A Linkage Map

AB – 30%

AC – 15%

AD – 10%

BC – 45%

BD – 20%

CD – 25%

C A D B (or B D A C)

Only Tells Us Sequence

Other methods to determine the actual location

Sex Chromosomes

X and Y chromosomes act like homologous chromosomes– but aren't really

Very little crossing over

Contain different genes

x

y

Genes on the X and Y Chromosomes Have Unique

Patterns of Inheritance Sex chromosomes

contain many genes not related to sex

Most are found on the X chromosome

Very few genes on the Y chromosome

Recessive Sex-Linked Traits

Daughters are only affected if they receive the allele from both parents

Males need only receive 1 copy of the affected X- chromosome from their mother

Recessive Sex-Linked Traits

Color-blindness Hemophilia Muscular dystrophy Fragile X-syndrome Many, many more All of these

disorders are MUCH more common in males

Punnett Squares with Sex-Linked Genes

•XA represents the X chromosome with the dominant allele

•Xa represents the X chromosome with the recessive allele

•Y represents the Y chromosome

Female carrier

Affected Male

If a color blind female mates a normal vision male…

• What % of their male offspring will be color blind?

• Female offspring?

If a color blind male mates a carrier female…

• What % of their male offspring will be color blind?

• Female offspring?