The Chromosomal Basis of Inheritance

Connecting Mendel's Work to Chromosomes The Chromosomal Basis of Inheritance

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 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!

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 Grey – Vestigial Black - 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

0% means genes are inherited together 50% means completely separately Genetic Recombination • Recombinants have new genotypes • Frequency of recombination is the % of offspring that have new genotypes

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 • 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?

The Chromosomal Basis of Inheritance