Chapter 15: The Chromosomal Basis of Inheritance

Law of Independent AssortmentAlleles 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.

b+vg+ b+vg bvg+ bvg

bvgb+bvg+vg b+bvgvg bbvg+vg 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 problems1. 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