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Genetics > Sex Linkage and Pedigrees > Flashcards

Sex Linkage and Pedigrees Flashcards

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Biology 101 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
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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)
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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
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4
Q

Environmental Sex Determination

A
  • Limpet’s position in the stack
  • Temperature in reptiles
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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
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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

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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
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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
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9
Q

Non-disjunction in meiosis I

A
  • XXX (Triple X)
  • XXY (Klinefelter syndrome)
  • X (Turner syndrome)
  • Y (non-viable)
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10
Q

Non-disjunction in meiosis II

A
  • XXX (Triple X)
  • XXY (Klinefelter syndrome)
  • X (Turner syndrome)
  • Y (non-viable)
  • XX (female)
  • XY (male)
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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
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12
Q

X-linked characteristics

A
  • Through performing crosses
  • The results of reciprocal crosses are consistent with x-linked inheritance
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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
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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
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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
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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
  1. all affected are rr
  2. 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
  3. if two unaffected mate and have an affected child, both parents must be Rr
  4. Recessive outsider rule: outsiders are those whose parents are unknown. in a recessive autosomal pedigree, unaffected outsiders are assumed to be RR homozygous
  5. 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
  6. 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

Genetics (3 decks)

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