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17.1 Many organisms have a distinctive pair of chromosomes, often called the X and Y chromosomes, that differ between the sexes and show different patterns of inheritance in pedigrees from other chromosomes.
In humans and other mammals, XX individuals are female and XY individuals are male. page 346
The human X and Y chromosomes are different lengths and contain different genes, except for small regions of homology that allow the two chromosomes to pair in meiosis. page 347
Segregation of the X and Y chromosomes during male meiosis results in half of the sperm receiving an X chromosome and half a Y chromosome so that random union of gametes predicts a 1:1 female:male sex ratio at the time of fertilization. page 347
17.2 X-
Thomas Hunt Morgan studied a mutation in the fruit fly Drosophila melanogaster that resulted in fruit flies with white eyes rather than wild-
In a cross of a normal red-
This pattern of inheritance is observed because the gene Morgan studied is located in the X chromosome. The nonmutant w+ allele is dominant to the mutant w– allele, and the gene is present only in the X chromosome and not in the Y chromosome. page 348
X-
Calvin Bridges observed rare fruit flies that did not follow the usual pattern for X-
In humans, X-
17.3 In genetic linkage, two genes are sufficiently close together in the same chromosome that the combination of alleles present in the chromosome tends to remain together in inheritance.
Genes that are close together in the same chromosome are linked and do not undergo independent assortment. page 353
Recombinant chromosomes result from crossing over between genes on the same chromosome and show a nonparental combination of alleles. page 354
Nonrecombinant chromosomes have the same configuration of alleles as one of the parental chromosomes. page 354
In genetic mapping, the observed proportion of recombinant chromosomes is the frequency of recombination and can be used as a measure of distance along a chromosome. A recombination frequency of 1% is 1 map unit. page 355
Gene linkage and mapping are used to identify the locations of disease genes in the human genome. page 357
17.4 Most Y-
In humans and other mammals, the Y chromosome contains a gene called SRY that results in male development. page 357
In Y-
Most Y-
17.5 Mitochondria and chloroplast DNA follow their own inheritance pattern.
Mitochondria and chloroplasts have their own genomes, which reflect their evolutionary history as free-
Mitochondria in humans and other mammals show maternal inheritance, in which individuals inherit their mitochondrial DNA from their mother. page 359
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Because mitochondrial DNA does not undergo recombination and is maternally inherited, it can be used to trace human ancestry and migration. page 360
Explain how the human X and Y chromosomes can pair during meiosis even though they are of different lengths and most of their genes are different.
The X and Y chromosomes can pair during meiosis through regions of homology located near the tips of the chromosomal arms.
Describe the biological basis for the 1:1 ratio of males and females at conception in mammals.
Meiosis in the mammalian egg cell results in X-
For a recessive X-
When an affected male mates with a homozygous nonmutant female, all of the sons are normal (because they receive their X chromosome from their mother), but all of the daughters are heterozygous (because they must receive their father’s X chromosome). When one of these heterozygous daughters mates with a normal male, half of the sons are affected (they inherit their grandfather’s X-linked allele) and half are normal (they inherit their grandmother’s X-linked allele); as for the daughters, while all of the daughters are phenotypically unaffected, half are heterozygous (they inherit their grandfather’s X-linked allele), whereas half are homozygous nonmutant (they inherit their grandmother’s X-linked allele).
Explain why linked genes do not exhibit independent assortment.
Linked genes do not exhibit independent assortment because they are located sufficiently close together on the same chromosome that crossing over is unlikely to occur between them and they segregate into gametes together.
Describe how recombination frequency can be used to build a genetic map.
The closer, or more tightly linked, that two genes are to each other, the smaller the frequency of recombination because it is less likely that a crossover event would take place in the interval between them. The further two genes are from each other, the greater the frequency of recombination because there would be a greater chance that a crossover event will happen in the interval between the genes. The frequency of recombination can be used as a measure of distance between the genes.
Describe the pattern of inheritance expected from a Y-
For a Y-
Describe the pattern of inheritance expected from a gene present in mitochondrial DNA in a human pedigree.
In humans, a gene present in mitochondrial DNA is transmitted from the mother’s egg cell to the offspring. Both males and females can show the trait, all offspring from an affected female show the trait, and males never transmit the trait to their offspring.
Explain how Y-
Y-linked genes show complete linkage, which means that its sequences are not exchanged for others through crossing over and mutations can accumulate in a single line of inheritance. Mitochondrial DNA also does not undergo recombination and is maternally inherited. Because each hereditary lineage of Y chromosomes and mitochondria is separate from every other lineage, they can be used to trace an individual’s ancestry.