Sex-linked inheritance/pedigree Flashcards
Who was Thomas Hunt Morgan and what did he study?
- Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome
- Morgan spent a year looking for variations among the flies he was breeding
- Unexpectedly, Morgan found that the eye color gene was inherited in different patterns by male and female flies
- During his studies, Morgan discovered a single male fly with white eyes instead of usual red
o Most common character phenotype is wild type (i.e., red eyes)
o Less common character phenotype is mutant (i.e., white eyes) - In one experiment, Morgan mated male flies with white eyes with female flies with red eyes:
o F1 generation all had red eyes
Therefore, the real interesting finding by Morgan came when he performed experiments that involved the matings of F1 flies to make the F2 generation
o Males and females from F1 generation mated to give rise to the F2 generation, showed 3 red : 1 white eye ratio (males had white eyes)
o White-eyed trait appeared only in males
o All females and half the males had red eyes - Morgan concluded that fly’s eye color was linked to its biological sex
Why did Morgan Hunt use drosophilia?
Drosophila melanogaster, a fruit fly species that eats fungi on fruit
o Fruit flies are prolific breeders and have a generation time of two weeks
o Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males)
What are unlinked genes?
Genes that are not near each other on the same chromosome (or found on separate chromosomes), are called unlinked genes
– Such genes are not inherited together often as a set
– For genes that are far apart on the same chromosome, there is frequent crossing over that occurs (at prophase I) between homologous chromosomes
* Unlinked genes follow Mendel’s law independent assortment
* all possible combination of maternal and paternal alleles are equally likely (25%) in each gamete/four daughter cells from meiosis
What are linked genes?
Genes located near each other on the same chromosome are called linked genes since tend to travel together during meiosis and fertilization
– Such genes are often inherited as a set as crossing over between homologous chromosomes is very rare between genes that are very close together
- recombinant chromosomes do form but not very likely (mix of maternal/paternal alleles)
* Linked genes do not follow Mendel’s law independent assortment
- gametes made most likely have the same allele combination of parents (48% maternal or 48% paternal), and only small likelihood of recombinant gametes (2% and 2%)
What is incomplete linkage (what study did thomas morgan hunt do to discover this)?
Morgan did other experiments with fruit flies to see how linkage affects inheritance of two different characters (so looking at two genes)
* Morgan observed this linkage when he followed inheritance of characters for body color and wing size (both traits are autosomal):
o Wild-type body color is gray (b+) and mutant is black (b)
o Wild-type wing size is normal (vg+) and mutant has vestigial wings (vg)
* Morgan crossed females heterozygous for both genes (b+bvg+vg) with mutant males homozygous for both genes (bbvgvg)
- According to independent assortment (i.e., unlinked genes), this should produce four phenotypes in a 1:1:1:1 ratio
- Surprisingly, when Morgan performed the experiment, he did not get a 1:1:1:1 ratio
– He observed a large number of flies were wild-type (gray-normal) and double-mutant (black-vestigial) among the offspring, but also there were a small number of flies with recombinant phenotypes (gray-vestigial and black-normal)
– The results of Morgan’s testcross for body color and wing shape did not conform to either unlinked or completely linked genes
– If both genes are unlinked, we should expect to see a a 1:1:1:1 phenotypic ratio (1 gray-normal: 1 black-vestigial: 1 gray-vestigial: 1 black-normal)
– If both genes are completely linked, we should expect to see a 1:1:0:0 ratio with only parental phenotypes among offspring (1 gray-normal: 1 black- vestigial: 0 gray-vestigial: 0 black-normal)
- Most of the offspring had parental phenotypes (gray-normal and black-vestigial) suggesting linkage between genes
* However, 17% of flies were recombinants (i.e., gray-vestigial and black-normal), suggesting incomplete linkage
- This means that genes for body color and wing shape are located very close to each other on the same chromosome (but the short distance between these genes still allows for crossing over to occur – even though it would be very seldom)
What are sex-linked genes and how are males mostly affected?
A gene located on either sex chromosome is called a sex-linked gene
* Human X chromosome contains approximately 1100 genes, whereas the Y chromosome contains only 78 genes
* Therefore, most sex-linked genes are found on the X chromosome
If sex-linked trait is due to a recessive allele, a female will have this trait only if homozygous (heterozygous females will be carriers)
* Since males have only one X chromosome, any male receiving the recessive allele from his mother will express the sex-linked trait
– Therefore, males are far more likely to inherit sex-linked recessive disorders compared to females
* Several serious human disorders are sex-linked
What is Duchenne Muscular Dystrophy?
- Affected individuals rarely live past their early 20s (affects
1 in 3,500 males in USA)
o Disorder due to absence of an X-linked gene for a muscle
protein called dystrophin
– Dystrophin is part of a group of proteins (a protein complex) that work together to strengthen muscle fibers and protect them from injury as muscles contract and relax
o Characterized by progressive weakening of muscles
What is hemophilia?
- X-linked disease
Absence of one or more clotting factors (normally there are 13 blood clotting proteins that combine to form a clot)
o Individuals have prolonged bleeding because a firm clot forms slowly
o Bleeding in muscles and joints can be painful and lead to serious damage
o Individuals can be treated with intravenous injections of the missing protein
What is hypertrichosis?
- X-linked
Can be either congenital (present at birth) or acquired later in life
o Result in excessive or animal-like hair on both face and body
o Two distinct types: generalized hypertrichosis (which occurs over the entire body) and localized hypertrichosis (which is restricted to a certain area)
o Unfortunately, some of these people have been displayed in carnival sideshows with names such as “dog-boy” or the “bearded lady”
What is non-disjunction?
Pairs of homologous chromosomes do not separate normally during meiosis (i.e., at meiosis I or II)
o Gametes contain two copies or no copies of a particular chromosome
o As a consequence of non-disjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy
What is aneuploidy?
occurrence of extra or missing chromosomes due to non-disjunction, which leads to an unbalanced chromosome complement (if organism survives, aneuploidy typically leads to a distinct phenotype)
What is down syndrome?
One aneuploid condition, Down syndrome, is due to three copies of chromosome 21
* Although chromosome 21 is one of the smallest human chromosomes, it severely alters an individual’s phenotype in specific ways
* Most cases of Down syndrome result from non- disjunction during gamete production in one parent
* Frequency correlates with age of mother (***father)
– This may be linked to some age-dependent abnormality in the spindle checkpoint during meiosis I, leading to non-disjunction
What are examples of non-disjunction of sex-chromosomes?
- Non-disjunction of sex chromosomes produces a variety of aneuploid conditions in humans:
- Klinefelter’s syndrome (XXY)
- Monosomy X or Turner’s syndrome (X0 or X-)
- Trisomy X (XXX)
- Jacob’s syndrome (XYY)
What is trisomy X?
XXX (three X chromosomes)
- Healthy females with mild to severe symptoms of developmental delays (learning disabilities, delayed speech/ language development)
– Capable of reproducing (limited)
What is Jacob’s syndrome?
- XYY
- Occurs when male inherits two Y chromosomes from father instead of one
– Often are taller than average (which becomes apparent at age of five or six) with an average final height approximately 7 cm (3”) above expected final height
– Often have below normal intelligence (learning disabilities have been reported in up to 50% of cases, most commonly speech delays and language problems…reading difficulties are common due to an increased incidence of dyslexia)
– Males that are XYY are still able to have children
– At one time (~1970s), it was thought that these men were likely to be criminally aggressive, but this hypothesis has been disproven over time
