Sex Linkage and Pedigrees Flashcards
1
Q
Sex linked inheritance - sex determination
A
- in mammals and some fish, plants, insects, reptiles, sex is determined by the X and Y chromosomes
- XX = female; XY = male
- human Y chromosome <200 genes
- human X chromosome ~ 1100 genes
2
Q
Other sex determination systems
A
- in birds, snakes, some fish and some insects: Z/W system
- some insects: XX-XO system; XX = female, XO = male
- Haploidiploidy system; haploid = male, diploid = female (bees, wasps, ants)
3
Q
Sex defined
A
- Heterogametic sex: the gender that produces two different gametes with respect to the sex chromosomes
- Homogametic sex: the gender that produces gametes that are the same with respect to the sex chromosomes
- Hemizygous: possessing a single allele of a locus
4
Q
Environmental Sex Determination
A
- Limpet’s position in the stack
- Temperature in reptiles
5
Q
Genic Sex-determining System
A
- some plants and protozoans, there are no sex chromosomes
- genic sex determination: genotypes at one or more loci determine the sex of an individual plant or protozoan
- no sex chromosomes, only the sex-determining genes
6
Q
Pseudoautosomal sex chromosomes
A
The X and Y chromosomes are homologous only at pseudoautosomal regions, which are essential for X-Y chromosome pairing in meiosis in the male
7
Q
Human Males
A
- SRY is the primary Sex-determining Region Y gene in mammals - causes gonads to become testes
- other genes are involved in fertility and development of sex characteristics
8
Q
The Role of Sex Chromosomes
A
- the x chromosome contains genetic information essential for both sexes; at least one copy of an X is required
- a single Y, even in the presence of several X, still produces a male phenotype
- the absence of Y results in a female phenotype
9
Q
Non-disjunction in meiosis I
A
- XXX (Triple X)
- XXY (Klinefelter syndrome)
- X (Turner syndrome)
- Y (non-viable)
10
Q
Non-disjunction in meiosis II
A
- XXX (Triple X)
- XXY (Klinefelter syndrome)
- X (Turner syndrome)
- Y (non-viable)
- XX (female)
- XY (male)
11
Q
Sex-linked characteristics
A
- sex-linked traits are determined by genes on the sex chromosomes
- X-linked characteristics are found on the X chromosome
- Y-linked are found on the Y chromosome
- sex-linkage was first documented by Thomas Hunt Morgan’s research
12
Q
X-linked characteristics
A
- Through performing crosses
- The results of reciprocal crosses are consistent with x-linked inheritance
13
Q
X-linked colour blindness
A
- genes for red and green pigment are on the X chromosome in humans
- red-green colour blindness inherited as an X-linked recessive characteristic
- Xc = allele for red-green colour blindness
- X+ = allele for normal vision
- reciprocal cross results in different outcomes
- X-linked recessive
14
Q
Genotypes in sex-linked inheritance
A
- sex-linked dominant trait; expresses trait: male XAY, female XAXA or XAXa / no expression: male XaY, female XaXa
- sex-linked recessive trait; expresses trait: male XaY, female XaXa / no expression: male XAY, female XAXA or XAXa
- Most common mistakes:
- XY because the male does not show the trait - unaffected does not mean that the gene is not on the X
- XY A or a: if x-linked, no gene on the Y
15
Q
Dosage Compensation
A
- 2 X chromosome in females
- 1 X chromosome in males
- Females = double amount of protein
- Dosage compensation
- X chromosome in male fruit flies does double duty
- some worms: the gene activity 50% on both X chromosomes
16
Q
Barr Bodies and Lyon hypothesis
A
- Murray Barr identified Barr bodies in 1949
- Mary Lyon proposed in 1961 the now confirmed Lyon hypothesis showing that one X chromosome is inactivated in female cells
- most genes are inactivated but not all
17
Q
Dosage compensation
A
- mammals randomly inactivate one of the X chromosomes
- at x-linked loci, 50% of cells express one gene and 50% express the other
- this is passed on when cells replicate during mitosis so nearby cells express the same trait
18
Q
Calico Cats
A
- males can either be black or orange
- females can be black, orange, or calico
- descendants of each cell line have the same inactivated X, resulting in a mosaic. but different cell lines have different inactivated X
19
Q
Dosage compensation
A
- barr-body effect allows extra X chromosomes to be tolerated well
- regulated by specific loci, X inactivation center, Xist, produces RNA that coats and silences the extra X
20
Q
Genetic Counselling
A
- evaluate family history and medical records
- construct a pedigree
- genetic tests carried out if necessary
- helps understand personal risk or risk to other family members
- make decisions about family planning or management
20
Q
Why use pedigrees
A
- controlled breeding not possible
- small family size
- long generation times
- solution: use family trees to study inheritance of traits
21
Q
Application: Captive Breeding
A
- avoid inherited genetic diseases (similar to genetic counselling)
- avoid inbreeding
- breed desirable traits
22
Q
Pedigree - Symbols
A
- circle = female
- square = male
- diamond = not specified
- coloured in = affected
- not coloured in = unaffected
- crossed out = deceased
- half filled or dot = carrier
- Proband: the first affected member identified by geneticist
22
Q
Pedigrees - Definition
A
A pedigree is a genetic representation of a family tree that displays the inheritance of a trait or disease though several generations
The pedigree shows the relationships between family members and indicated which individuals express or silently carry the trait in question
23
Q
X-linked Dominant
A
- for rare conditions females are about 2x as likely to be affected than males. may be lethal in males and usually milder, but variable, in females
- affected males pass the gene to all of their daughters, who will be affected, and none to their sons
- sons and daughters of affected females have 50% chance of being affected
- less common that x-linked recessive
- heterozygous affected mothers pass the allele to half their sons and daughters
24
Q
X-linked Recessive
A
- males are more often affected than females
- affected males pass the gene to all of their daughters and none of their sons
- daughters of carrier females have a 50% chance to be unaffected carriers. Sons of carrier females have 50% chance to be affected
- affected males in the family are related to each other through carrier females
25
Q
Autosomal Recessive
A
- Autosomal recessive traits usually appear equally in males and females
- and tend to skip generations
- autosomal recessive traits are more likely to appear among progeny of related parents = consanguine mating
- when both parents are affected, child is affected
- unaffected/heterozygous parents may have affected children
- if the trait is rare most matings of unaffected and affected produce unaffected children
- both parents are heterozygous - 1/4 of offspring will be affected
26
Q
Genotypes and Probability - Autosomal Recessive
A
- if the mother is Rr and father is Rr then child has 1/4 chance of being rr
- probability: child is rr = 2/3 x 2/3 x 1/4 = 1/9 = 0.11 ~ 11% chance that the first child will have trait
27
Q
Assigning genotypes for recessive pedigrees
A
- all affected are rr
- if an affected person mates with an unaffected person, any unaffected offspring must be Rr heterozygotes, because that got an r allele from their affected parent
- if two unaffected mate and have an affected child, both parents must be Rr
- Recessive outsider rule: outsiders are those whose parents are unknown. in a recessive autosomal pedigree, unaffected outsiders are assumed to be RR homozygous
- children of RRxRr have a 1/2 chance of being RR and a 1/2 chance of being Rr. Note that any parents who have an rr child must be Rr
- unaffected children of RrxRr have a 2/3 chance of being Rr and 1/3 chance of being RR
28
Q
Autosomal Dominant
A
- autosomal dominant traits usually appear equally in males and females
- unaffected persons do not transmit the trait
- affected persons have at least one affected parent
- trait usually occurs in every generation
29
Q
Dominant Genetic Disorders
A
- caused by the dominant alleles
- if only one parents has one allele, 50% of the children will inherit the disease
30
Q
Conditional Probability
A
- in some cases, one or both parents has a genotype that is not completely determined. for instance, one parents has a 1/2 chance of being BB and a 1/2 of being Bb
- if the other parent is bb and this is a dominant autosomal pedigree
1. determine the probability of an affected offspring for each possible set of parental genotypes
2. combine them using the AND and OR rules of probability
31
Q
Multiple Alleles
A
- Red blood cells that do not react with the recipient antibody remain evenly dispersed. donor blood and recipient blood are compatible
- blood cells that react with the recipient antibody clump together. donor blood and recipient blood are not compatible
- type O donors can donate to any recipient: they are universal donors
- Type AB recipients can accept blood from any donor: they are universal recipients
32
Q
Y-linkage
A
- only observed in males
- most of the Y chromosome is “junk” so there is a high mutation rate
- these mutations are passed from father to son
- these site specific mutations can be used to track paternal and evolutionary lineages
- traits on the Y chromosome are only found in males, never in females
- Dominance is irrelevant: there is only 1 copy of each Y-linked gene (hemizygous)
- affected males pass the disease gene to all their sons and to none of their daughters
33
Q
Mitochondrial trait
A
- transmission occurs only through females
- note that only 1 allele is present in each individual, so dominance is not an issue
