Chapter 16: Inheritance of Sex Chromosomes, Linked Genes, and Organelles
features of X-linked inheritance
- 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.
Nondisjunction of autosomes in humans
- Trisomy = 21 chromosome pairs; down syndrome - Trisomy 12 = Patau syndrome - Trisomy 18 = Edward's syndrome
Common forms of nondisjunction of sex chromosomes in humans
- XXX: triple X syndrome - XXY: Klinrfelter syndrome - XYY = Jacob's syndrome - XO = Turner syndrome
For a recessive X-linked mutation, such as color blindness, what is the pattern of inheritance from an affected male through his daughters into her children?
- if mates with nonmutant homozygous female, then daughters will be heterozygous carriers & males will be normal (because they recieve x from mom) - if the heterozygous daughters mate with a normal male, half sons are affected & half aren't, half females are heterozygous & half are homo nonmutant
Alfred H. Sturtevant
- student of T.H. Morgan -1911 - generated first map of chromosome based on recombination frequency - discovered linkage through eye color and cross veins on wings -said that frequency of recombination could be used to determine the physical distance separating two genes on a chromosome
Color blindness is a recessive X-linked trait in humans. In a family where the mother is color-blind and the father is normal, the probability of their having a color-blind son is:
1
two important principles governing the inheritance of X-linked genes:
1. The phenotypes of the XX offspring indicate that a male transmits his X chromosome only to his daughters. In this case, the X chromosome transmitted by the male carries the white-eye mutation. 2. The phenotypes of the XY offspring indicate that a male inherits his X chromosome from his mother. In this case, the X chromosome transmitted by the mother carries the nonmutant allele of the gene.
Color blindness is a recessive X-linked trait in humans. In a family where the mother is heterozygous for color blindness and the father is color-blind, the probability of their having a color-blind daughter is:
1/4
For most genes on the human X chromosome, what percent of XY males with a mutant allele on the X chromosome will express the mutant
100%
measuring distance between genes EX:
357 + 341 + 52 + 45 = 795, and the number of recombinant chromosomes (w+cv−/Y and w−cv+/Y) is 52 + 45 = 97. The frequency of recombination between w and cv is therefore 97/ 795 = 0.122, or 12.2%, and this serves as a measure of the genetic distance between the genes
Aneuploidy
A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.
recombinant chromosomes
A chromosome created when crossing over combines the DNA from two parents into a single chromosome.
genetic map
A diagram showing the relative positions of genes along a chromosome.
chromosomes transmission
A male will ONLY transmit his X chromosomes to his XX daughter, and the male offspring inherit their x chromosomes from their mother
Criss-cross Inheritance Pattern
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
map unit
A unit of distance in a genetic map equal to the distance between genes resulting in 1% recombination
In fruit flies, what kind of eye color would be expressed in male progeny from a white-eyed female and a red-eyed male?
All males would have white eyes.
What is the relationship between genes, alleles, and chromosomes?
Alleles are different forms of same gene. Genes are linearly arranged on chromosomes. Chromosomes contain genetic material of cell i.e. DNA. So chemically alleles, genes, chromosomes are all DNA !
X-Linkage: Female Parent with recessive allele
Among progeny from a heterozygous carrier female, half the daughters are heterozygous carriers, and half the sons are affected.
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. w+cv− and w−cv+
Autosomes
Any chromosome that is not a sex chromosome
characteristic pedigree patterns of a trait encoded by a mitochondrial genome transmitted through maternal inheritance:
Both males and females can show the trait. All offspring from an affected female show the trait. Males do not transmit the trait to their offspring.
homologous chromosomes
Chromosomes that have the same sequence of genes and the same structure
Crossing over but no recombination
Crossing over does not always take place between genes of interest. This results in four nonrecombinant chromosomes Two ways: 1. These nonrecombinant configurations occur because crossing over occurs at the four-strand stage of meiosis (when each homologous chromosome is a pair of sister chromatids), but only two of the four strands (one sister chromatid from each homologous chromosome) are included in any crossover. 2. When two genes are close together in a chromosome, a crossover may occur not in the interval between the genes but at some other location along the chromosome Nonrecombinant chromosomes can also be the result of two crossover events occurring between two genes, in which the effects of the first crossover (creating recombinants) is reversed by a second crossover (re-creating nonrecombinants).
Linked
Describes genes that are sufficiently close together in the same chromosome that they do not assort independently and transmit together in inheritance. Nearby genes in the same chromosome show linkage. linked genes have recombination frequency of less than 50%
y-linked genes
Doesn't share homology with the x chromosome; cannot cross over with the x chromosome 1. Passed from father to son to grandson ex: SRY gene 2. females do not inherit or transmit 3. all sons will show trait
Most chromosomes have one or more crossovers that form between the homologous chromosomes
Human females average about 2.75 crossovers per chromosome pair, and human males average about 2.50
How is it possible that there are multiple different alleles in a population and yet any individual can have only two alleles?
In the population as a whole, there are many copies of each chromosome, so any gene can have multiple alleles present in the different copies. Any one individual can have only two alleles because their genes are inherited from the mother and the father, half and half, and there are two copies of any chromosome, and so any individual can have no more than two different alleles.
XY males
Less likely to survive after having reach reproductive maturity As age progresses the ratio is skewed towards females
For an X-linked recessive trait:
Male cannot be a carrier Both chromosomes must posses the recessive allele;female must be HOMOZYGOUS
Red-green color blindness is an X-linked recessive trait in humans. Martha has wildtype color vision. Her mother has color vision, and her father is color blind. Bill is color blind. What are the genotypes of Bill and Martha? If Bill and Martha have children, what proportion of boys and girls will be color blind
Martha would be heterozygous for the color blindness and Bill would have one x chromosome with the color blindness allele and a Y chromosome. Their possible offspring would be homozygous recessive female (color blind), heterozygous female (color vision), recessive X male (color blind), or dominant X male (color vision). This means that ½ of the boys and ½ of the girls would be colorblind
mitochondria normally show strictly maternal inheritance (mother)
Maternal inheritance is characteristic of mitochondrial diseases
types of inheritance of cytoplasmic organelles
Maternal inheritance, in which the organelles in the offspring cells derive from those in the mother. Paternal inheritance, in which the organelles in the offspring cells derive from those in the father. Biparental inheritance, in which the organelles in the offspring cells derive from those in both parents.
What is 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-bearing eggs only. In contrast, meiosis in the sperm cell results in a 1:1 ratio of female : male offspring.
1 : 1 ratio can be predicted based off of chromosome separation in...
Meiosis stage
How can mitochondrial DNA data be used to trace ancestry?
Mitochondrial DNA doesn't undergo recombination, so mutations can accumulate in a single line of inheritance, inherited maternally. Because each hereditary lineage of mitochondria is separate from every other lineage, mitochondria can be used to trace an individual's ancestry
Nondisjunction as evidence that genes are present in chromosomes
Normal chromosome separation yields expected progeny. (b) Nondisjunction yields exceptional progeny.
A man affected with a mitochondrial disease mates with a woman who does not have a mitochondrial disease. What is the probability that their offspring will be affected?
Offspring will not likely be affected because mitochondria of offspring come from mother, not father.
X chromosome
One of the sex chromosomes; a normal human female has two copies of the X chromosome; a normal male has one X and one Y chromosome. - DNA molecule is more than 150 Mb long - Chromosome has over 1,000 genes
Y chromosome
One of the sex chromosomes; a normal human male has one X chromosome and one Y chromosome. - about 50 Mb long - contains about 50 protein-coding genes
single nucleotide polymorphism (SNP)
One-nucleotide DNA sequence variation which one of the nucleotide pairs in the DNA is a G—C (mutant allele) base pair in some chromosomes and A—T (nonmutant) in others arise due to ancestry
Mitochondrial and chloroplast genomes often show uniparental inheritance.
Organelles are partitioned to the gametes along with other cytoplasmic components, and therefore their mode of inheritance in different species depends on how the gametes are formed, how much cytoplasm is included in the gametes, and the fate of the cytoplasm in each parental gamete after fertilization.
Sturtevant experiment
Parent generation: female homozygous for both nonmutant alleles (red eyes and normal veined wings) and male with both mutant alleles (white and crossed veins) F1: females were heterozygous and males were normal F2: Males had variations of phenotypes NOT in 1: 1: 1: 1: 1 ratio * Lack of proper ratio expected with independent assortment means the genes show linkage *
nonrecombinant
Progeny in which the alleles are present in the same combination as that present in a parent. w+cv+ or w−cv−
genetic mapping
SNPs associated with a mutant gene show where that gene is located in the genetic map. failure to find an association means that the SNP is not closely linked to the disease gene, and may in fact be in a different chromosome
Sturtevant: Drosophila
Sturtevant worked with male fruit flies that have an X chromosome carrying two recessive mutations. One is in the white gene (w) discussed earlier, which when nonmutant (w+) results in fruit flies with red eyes and when mutant (w−) results in fruit flies with white eyes. The other recessive mutation is in a gene called crossveinless (cv), which when nonmutant (cv+) results in fruit flies with tiny crossveins in the wings and when mutant (cv−) results in the absence of these crossveins. Sturtevant crossed this doubly mutant male with a female carrying the nonmutant forms of the genes (w+ and cv+) in both X chromosomes. He saw that the offspring consist of phenotypically wild-type females that are heterozygous for both genes, and phenotypically wild-type males. When these are crossed with each other, the female F2 progeny do not tell us anything because they are all wild type; each female receives the w+cv+ X chromosome from her father and therefore has red eyes and normal crossveins. In the male F2 progeny, however, the situation is different: each male progeny receives its X chromosome from the mother and its Y chromosome from the father, and so the phenotype of each male immediately reveals the genetic constitution of the X chromosome that the male inherited from the mother.
Sturtevant Experiment
Taking this idea a step further, Sturtevant reasoned that if one knew the frequency of recombination between genes a and b, between b and c, and between a and c, then one should be able to deduce the order of these three genes along the chromosome. The logic is that the two genes showing the highest frequency of recombination are farthest apart, and the third gene is in the middle. He also predicted that, if the order of genes were known to be a-b-c and if the genes were close enough, the frequency of recombination between a and c should equal the sum of the frequency of recombination between a and b and the frequency of recombination between b and c.
recessive alleles in the X chromosome are expressed in males because
The Y chromosome does not contain the wild-type allele
Wild Type
The most common allele, genotype, or phenotype present in a population; nonmutant.
A pedigree illustrates a mother carrying a trait, but the trait does not show up again in the pedigree till 3 generations later, in both a boy and a girl. Based on the pattern of affected individuals, what is the most likely mode of inheritance for Phenylketonuria?
The most likely explanation for this would be that Phenylketonuria is a recessive trait and the offspring reproduced with partners who had dominant alleles. It would come out within this last generation due to the shared family history of the two parents. Not all of the individuals are affected because they must carry both recessive alleles to express the gene.
haplotype (haploid genotype)
The particular combination of alleles present in any defined region of a chromosome.
Which of the answer choices is true of a cross involving a sex-linked gene but not of a cross involving an autosomal gene?
The phenotype outcomes of females and males can be different.
Sturtevant results
The results supported Sturtevant's hypothesis and showed that genes could be arranged in the form of a genetic map, depicting their linear order along the chromosome, with the distance between any pair of genes proportional to the frequency of recombination between them. Across sufficiently short regions, the frequencies of recombination are additive.
Why don't linked genes exhibit independent assortment?
They are too close on the same chromosome so crossing over is unlikely to occur & they segregate into gametes together
Why didn't Mendel discover the nature of sex chromosomes in his pea plants?
When Mendel first began his study of pea plants he was mostly looking for evidence of genotype through the expression of phenotype. When studying the pea plants all Mendel could see was the traits that they showed physically and then study how they passed generationally, which ultimately led to the discovery of traits that were passed down by genes from parents to off spring. Though of course there was an understanding of how reproduction occurred in the pea plants it was not possible with the technology and study he had access to to determine how certain traits may have been passed within the parental or maternal genes. Also, since pea plants have both reproductive organs occurring naturally, without recreating every level of experiment exactly the same but with different pollination techniques it would have been impossible to see a difference in ratio of offspring. Essentially the test subjects prevented Mendel from discovering the nature of sex chromosomes.
Reciprocal crosses do not produce the same types and numbers of progeny.
When a color-blind man mates with a woman who is not color blind, all of the sons and daughters have normal color vision. However, in the reciprocal cross, when a color-blind woman mates with a man who is not color blind, all of the daughters have normal color vision but all of the sons are color blind.
Meiosis I nondisjunction
X and Y do not separate
sex chromosomes
X&Y One of the 23 pairs of chromosomes in the human, unmatched and contains genes that will determine the sex of the individual.
How can the Y chromosome be used to trace ancestry?
Y-linked genes show complete linkage, which means that sequences in the Y chromosome are not exchanged for others through crossing over & mutations can accumulate in a single line of inheritance. Because each hereditary lineage of Y chromosome is separate from every other lineage, they can be used to trace ancestry - regions at the tips in which the X and Y chromosomes share homology is only about 6% of entire Y chromosome - 94% consists of linked sequences because that portion doesn't crossover - because of this, each hereditary lineage of Y chromosomes is separate from every other lineage - As mutations occur along Y chromosome, they are completely linked to any past mutations present and also completely linked to any future mutations - mutations therefore accumulate, allowing sequences to reconstruct
Is it possible for an unaffected female to have female offspring with red-green color blindness?
Yes. a woman whose father is color blind must be heterozygous mutant allele. If she has children with a man who is color blind, then half of the female offspring are expected to be homozygous mutant and therefore color blind.
Segregation of the sex chromosomes in meiosis and random fertilization result in
a 1 : 1 ratio of female : male embryos. primary sex ratio
If an XY male carries a mutation on his X chromosome, _______________ will receive the mutant X.
all of his daughters
Nondisjunction
can occur during anaphase 1 or 2 or meiosis: can have no sex chromosome or an extra
region of homology between X and Y chromosomes
consist of about 2.7 Mb of DNA near the tip of the short arm and about 0.3 Mb of DNA near the tip of the long arm
reciprocal crosses
crosses of male and female or the same genotype yield the same phygoney
Which of the statements is least likely to produce recombinant chromosomes?
crossovers between sister chromatids
X-linked inheritance
discovered through studies of male fruit flies with white eyes. by Thomas Hunt Morgan in 1910
A wild-type fruit fly:
exhibits the most common phenotype in the population.
Meiosis II nondisjunction
failure of sister chromatids to separate
The _____________ two loci are on a chromosome, the more likely they will be separated by crossover events during gamete formation, and thus the frequency of recombination between loci on a chromosome can be used to measure the _______________ between those loci.
farther apart; genetic distance
Gametes
females produce 1 type of gamete (X) Males produce 2 types of gametes (X and Y)
How can recombination frequency be used to build a genetic map?
frequency of recombination can be used as a measure of distance between the genes. - further two genes are apart, greater the frequency of recombination
Mitochondria
generate ATP for chemical energy mutations affect energy production either from lack of ATP production or energy production that is toxic to cell or causes damage to mitochondria
A normal female who carries a recessive X-linked allele for hemophilia will pass it on to:
half of both her sons and daughters.
random fertilization
half of the fertilized eggs are expected to be chromosomally XX (and therefore female) and half are expected to be chromosomally XY (and therefore male).
In some cells undergoing meiosis no crossover takes place between the genes
in which case all the resulting chromosomes are nonrecombinant and all alleles in each chromosome are completely linked only 223 = 8.4 million types of reproductive cells in this case
one reason why mitochondrial inheritance is strikingly different from nuclear inheritance
mitochondrial DNA is maternally inherited
O
no sex chromosomes
Nonmutant and mutant
non = + mutant = -
Crossover in most cells
occurs in the larger of regions of the chromosomes
progeny
offspring
primary sex ratio
proportion of males to females at time of conception 1 : 1 ratio- (XX to XY in punnett square)
secondary sex ratio
reflects proportion of each sex born - Slightly skewed towards males - U.S.: 100 F to 105 M by reproductive maturity the ratio becomes more equal to 1 : 1
Y chromosome haplotypes
set of dna variations and mutations that tend to be inherited together Any or all mutations may coexist within a population
XX females
slightly less likely to survive at birth, unknown cause
personal gnome
tell you about genetic risk factors and genetic ancestry
genes that are close together in the same chromosomes
tend to be transmitted together
Recessive alleles in the X chromosome are expressed in males because:
the Y chromosome does not contain the wild-type allele.
Inheritance of genetic materials found in organelles
the fact that they reside outside of the nucleus affects how DNA is passed down to daughter cells
Sturtevant hypothesis
the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
the closer two genes are to each other
the less likely crossing over will occur between them & lower the recombination frequency (i.e. expect only nonrecombinant chromosomes) frequency for all genes ranges from 0% to 50% because only 1 chromatid from each chromosome crosses
frequency of recombination
the proportion of recombinant chromosomes among the total number of chromosomes observed can be used a measure between the physical distance between linked genes
SRY gene (male determining gene)
the sex determining region of the Y chromosome in males. Encodes a protein that is the trigger for male development, male structures complete their development and female structures degenerate - male determining gene
When sperm and eggs unite
there is a 50% it is an XX zygote and a 50% of XY zygote
In humans, fruit flies, and many other organisms, most of the genes on the X chromosome are not found on the Y chromosome.
true
A male with an X-linked recessive trait will have heterozygous (carrier) daughters and normal sons.
true mutation on the X chromosome goes to the daughters in the punnett square
Crossing over results in
two non and two recombinant gametes half the chromosomes are non and the other half are recombinant this is because the crossing over only occurs in one on the chromatids in a chromosomes, and between two nonsister chromatids
How do the events of meiosis I promote the production of new combinations of alleles?
via both crossing over between homologous chromosomes & independent assortment Feedback: Correct. Independent assortment during anaphase I and crossing over between homologous chromosomes (nonsister chromatids) in prophase I produce genetic recombinants through recombination.
crossveinless (cv)
when nonmutant (cv+) results in fruit flies with tiny crossveins in the wings and when mutant (cv−) results in the absence of these crossveins
