Chapter 15: The Chromosomal Basis of Inheritance

1. Chapter 15: The Chromosomal Basis of Inheritance

2. Law of Independent Assortment Alleles on nonhomologous chromosomes separate independently during gamete formation.

3. Thomas Hunt Morgan • In early 1900’s used fruit flies and figured out the concept of linked genes • Symbols + = wildtype (normal) • For example, red eyes in fruit flies are the wildtype and white eyes are “mutants”. • Used w+ for red and w for white • Linked genes – genes that are on the same chromosome • Punnett squares from prior chapter assume that genes are NOT linked! • Linked genes change the possible offspring because linked genes change the gametes. Independent assortment will not affect linked genes. • Crossovers can affect linked genes!

4. Examples • For fruit flies b+ = gray body (wildtype) b = black body vg+ = normal wings (wildtype) vg = vestigial wings (small) • If the wing gene and body color genes are not linked then what results are expected from the cross below? Cross: b+bvg+vg x bbvgvg

5. If you actually perform the cross on the prior slide and you do not get the expected 1:1:1:1 ratio in the offspring, this indicates gene linkage and possibly crossovers!

6. So if genes are linked…. • Mommy makes fewer sex cells (only 2 instead of 4) because the b+ and vg+ stay together in her eggs. The Punnett square and possible offspring would look like this:

7. Of the genes are linked and crossovers occur… • A crossover occurred between mommy’s homologous chromosomes, which switched the vg and vg+ alleles.

9. The Punnett square would have to include 4 different possible sex cells for mommy (the non-crossovers and the crossover scenarios). The bold genotypes are the result of crossovers. • This Punnett square looks like the one for no linkage EXCEPT that the ratio would not be 1:1:1:1. • The bolded genotypes from the crossover gametes would have a different frequency than 25%. • The farther apart two genes are on a chromosome, the higher the probability of a crossover occurring.

12. Mapping Chromosomes • Where on the chromosome is the gene located? • Give the following information: • Crossover frequencies (recombination frequencies) • b-vg = 17% • b-cn = 9% • cn-vg = 9.5%

14. Sex-linked genes • Sex-linked genes are genes on the sex chromosomes • Sex chromosomes determine the gender in some species • In humans, XX is female and XY is male. • The Y chromosome is much smaller and does not contain all of the genes that the X does. • Males determine the sex of a child. • Sex-linked recessive traits are more common in males than in females. Why? • In females, one of the X’s must be inactivated and is called the Barr body. The one that is inactivated is random. A female will have a mosaic of two types of cells: those with the active X derived from the father and those with the active X derived from the mother.

18. Sex-linked problems • Cross a woman who is heterozygous for hemophilia with a man who is normal. Hemophilia is sex-linked and recessive. What are the possible offspring? Include gender in your answer.

19. In fruit flies, white eyes are recessive and sex-linked while red eyes are dominant. Cross a red-eyed female (whose father had white eyes) with a white-eyed male. What are the possible offspring? Include gender in your answer.

20. Sex-linked Pedigrees

21. Chromosome disorders • Nondisjunction – when a pair of chromosomes does not separate correctly in meiosis causing gamete to have too few or too many chromosomes • XXY – Klinefelter syndrome • Male born sterile, possible slight mental retardation, overdeveloped breasts with higher risk of breast cancer • XYY – Taller male • XXX – Normal female • X – Turner syndrome • Female born sterile, short, possible slight mental retardation, delayed puberty • Down syndrome – extra 21st chromosome

26. Breakage of Chromosome Structure • Duplication – fragment attached to a sister chromosome • Translocation – a fragment of a chromosome breaks off and joins another chromosome (not crossover) • Example – Chronic myelogenous leukemia results from exchange of large part of chromosome 22 with small piece of chromosome 9. This leads to a distinctly different (short) chromosome 22 called Philadelphia chromosome. • Inversion – fragment of a chromosome reattaches but in reverse orientation • Deletion – chromosomal fragment lacking a centromere is lost • Example - Cri du chat results from the deletion of piece of chromosome 5 • Causes mental retardation, small head, and cries like a cat

28. X-inactivation and calico cats • Gene for fur color is sex-linked with one allele yielding black and one orange. A female can end up with cells that have both active X with orange alleles or active X with black alleles. Males typically cannot be calico because they only inherit one X chromosome. • Genomic imprinting - certain genes can be imprinted depending on whether the gene resides in a male or female. This means that the same gene may have different effects depending on gender. In gamete forming cells, genomic imprint is erased and re-imprinted according to gender of the individual. • Example – in mice, paternal gene for Igf2 is expressed and the maternal allele is not. Igf2 is insulin-like growth factor. • Heterozygous mice with mutant allele from father = dwarf • Heterozygous mice with normal allele from father = normal size