LIFESCI 7B Lecture Notes - Lecture 2: Y Chromosome, Haemophilia A, Nondisjunction
10 Jun 2018
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17.1: The X and Y Sex Chromosomes
● Inheritance of certain traits via X/Y chromosomes isn’t consistent with reciprocal crosses (switching
genotypes w/ gender)
Segregation of the sex chromosomes predicts a 1:1 ratio of females to males. copy
● Males produce half Y-bearing gametes and half X gametes; females carry only X gametes
○ 50/50 chance of male or female based on this cross (XX x XY)
■ After birth, slightly higher ratio of males due to higher survival rate from conception
○ Sex determination independent; family’s history/previous children has no effect
In many animals, sex is genetically determined and associated with chromosomal differences.
● sex chromosome Any of the chromosomes associated with sex.(X and Y)
○ X chromosome significantly larger than Y, carries many genes Y doesn’t
○ Only homologous at very small region at the tip, allowing these chromosomes to pair during
meiosis
■ During meiosis crossover occurs at larger regions of these chromosomes, occurs
between NONSISTER CHROMATIDS
○ autosome Any chromosome other than the sex chromosomes.
● relative size and gene content (and region of homology) of the X and Y chromosomes differ greatly
among species
○ Some have similar sized XY, some have one X vs two X’s, some have female heterogametey
17.2: Inheritance of Genes in the X Chromosome
● X-linked gene A gene in the X chromosome.
X-linked inheritance was discovered through studies of male fruit flies with white eyes.
● wild type The most common allele, genotype, or phenotype present in a population; nonmutant.
Genes in the X chromosome exhibit a “crisscross” inheritance pattern.
● crisscross inheritance A pattern in which an X chromosome present in a male in one generation is
transmitted to a female in the next generation, and in the generation after that can be transmitted back
to a male.
X-linkage provided the first experimental evidence that genes are in chromosomes.
● nondisjunction The failure of a pair of chromosomes to separate normally during anaphase of cell
division.
○ Results in eggs carrying either 2 x chromosomes or none
■ Could lead to XXY females (fertile) or XO (sterile) males
■ XXX or OY not seen; assumed not survived
● O=absence of chromosome
○ Can take place in females AND males
○ For rare X-linked traits, most of the affected individuals are males.
● In humans, Y chromosome required for male fertility and development
Genes in the X chromosome show characteristic patterns in human pedigrees.
● Affected individuals are usually males because males need only one copy of the mutant gene to be
affected, whereas females need two copies to be affected.
● Affected males have unaffected sons because males transmit their X chromosome only to their
daughters.
● A female whose father is affected can have affected sons because such a female must be a
heterozygous carrier of the recessive mutant allele.
○ hemophilia A trait characterized by excessive bleeding that results from a recessive mutation in
a gene encoding a protein necessary for blood clotting.
17.3 Genetic Linkage and Recombination
Nearby genes in the same chromosome show linkage.
find more resources at oneclass.com
find more resources at oneclass.com
● linked Describes genes that are sufficiently close together in the same chromosome that they do not
assort independently.
○ Tend to be transmitted together
○ Could be on chromosome or autosome
○ Not x-linked gene, which is just a gene on the x chromosome
● nonrecombinants Progeny in which the alleles are present in the same combination as that present in
a parent.
● recombinant An offspring with a different combination of alleles from that of either parent, resulting
from one or more crossovers in prophase I of meiosis.
○ crossover The physical breakage, exchange of parts, and reunion between non-sister
chromatids.
■ Results in two recombinants and two nonrecombinants
● Nonrecombinants can also happen from two crossover between two genes, the first crossover (creating
recombinants) is reversed by a second crossover (re-creating nonrecombinants).
○ Happens during four strand stage of meiosis
The frequency of recombination is a measure of the genetic distance between linked genes.
● frequency of recombination The proportion of recombinant chromosomes among the total number of
chromosomes observed.
○ Closer genes=more tightly linked=less likely to crossover and produce recombinants
○ distance between genes is not measured directly by physical distance between them, but rather
by the frequency of recombination
○ Crossover ranges 0% (never) to 50% (almost always)
■ 50% because crossover happens at 2/4 chromosome strands; having two recombinant
and two nonrecombinant (linked genes very far apart) 50m.u.
● Recombination: creates new allele combos for each generation and ensuring genetic uniqueness of
individuals
Genetic mapping assigns a location to each gene along a chromosome.
● genetic map A diagram showing the relative positions of genes along a chromosome.
○ Distances from frequency of recombination used to construct this map
○ map unit A unit of distance in a genetic map equal to the distance between genes resulting in
1% recombination.
● With closer (<15mu) distances, distances between genes can be additive (distances between adjacent
genes can be added to get the distance between the genes at the ends)
○ greater distances (more than 15 map units), two or more crossovers between the genes may
occur in the same chromosome (recombinant and then reverse),
● independent assortment is observed for genes that are far apart in the same chromosome as well as
for genes in different chromosomes
Genetic risk factors for disease can be localized by genetic mapping. copy
● Association between disease gene and specific SNP (type of DNA marker) as a result of recombination
○ Shows whether or not an SNP (certain gene) is linked or possibly on another chromosome
○ Helps identify diseased gene on genetic map
17.4: Inheritance of Genes on the Y Chromosome
● SRY=Sex detertermining region in the Y chromosome; gene that triggers male development
Y-linked genes are transmitted from father to son to grandson.
● Y-linked gene A gene that is present in the region of the Y chromosome that shares no homology with
the X chromosome.
○ Mutations in these genes can result in male infertility and low sperm count
● Y-linked gene pedigrees:
○ Only males are affected with the trait.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Inheritance of certain traits via x/y chromosomes isn"t consistent with reciprocal crosses (switching genotypes w/ gender) Segregation of the sex chromosomes predicts a 1:1 ratio of females to males. copy. Males produce half y-bearing gametes and half x gametes; females carry only x gametes. 50/50 chance of male or female based on this cross (xx x xy) After birth, slightly higher ratio of males due to higher survival rate from conception. Sex determination independent; family"s history/previous children has no effect. In many animals, sex is genetically determined and associated with chromosomal differences. Sex chromosome any of the chromosomes associated with sex. (x and y) X chromosome significantly larger than y, carries many genes y doesn"t. Only homologous at very small region at the tip, allowing these chromosomes to pair during meiosis. During meiosis crossover occurs at larger regions of these chromosomes, occurs between nonsister chromatids.
