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Download Chapter 15: The Chromosomal Basis of Inheritance
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Chapter 15: The Chromosomal Basis of Inheritance Law of Independent Assortment Alleles on nonhomologous chromosomes separate independently during gamete formation. • 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! 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 • 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! 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: Of the genes are linked and crossovers occur… • A crossover occurred between mommy’s homologous chromosomes, which switched the vg and vg+ alleles. • 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. bvg b+vg+ b+vg b+bvg+vg b+bvgvg bvg+ bbvg+vg bvg bbvgvg • 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. 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% 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. Sex-linked problems 1. 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. 2. 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. Sex-linked Pedigrees 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 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 • 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