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:  1/4 th of offspring are b + bvg + vg  1/4 th are bbvg + vg,  1/4 th are b + bvgvg  1/4 th 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 genes AB- 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 X A represents the X chromosome with the dominant allele X a 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?