Mendelian Genetics, Punnett Squares, Sex-linked Dz Flashcards
Mendel’s contribution, basically
showed inheritance of traits follows particular laws, which were later named after him
allele
- alternative form of a gene (one member of a pair) that is located at a specific position (Locus) on a specific chromosome.
- Organisms have two alleles for each trait.
- Information about genotyple (which leads to phenotype)
- Examples:
- gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r).
Locus
Position of a gene along a chromosome
(allele is different form/nucleotide sequence of gene at a given locus)
hemoglobin allele
- Hb A vs Hb S
- Most have Hb A.
- Hb S differes by a single AA substitution in the ß-globin locus
- so ß-globin locus has two different alleles, one for A, one for S: it’s polymorphic
Polymorphism
locus containing 2+ alleles that occur w/appreciable frequency in a population is polymorphic or a polymorphism
e.g., Hb A & Hb S
heterozygous vs homozygous
- Background: humans are diploid organisms, each chromosome is represented twice, with one member of the chromosome pair contributed by the father and one by the mother
- At a given locus, one gene’s origin is paternal and one is maternal
- If both genes identical: homozygous at that locus
- “true breeding”
- example: O blood type (OO)
- If not identical: heterozygous
- AB blood type (A and B genes on pair of loci)
Phenotype
An organism’s expressed traits (green or yellow). In Mendel’s experiment, the F2 generation had a 3:1 phenotypic ratio of plants with green pods to plants with yellow pods.
Punnett square:possibilities of offspring for pair of homozygous dominant & homozygous recessive
If all offspring are heterozygous, you know parents were both homozygous: recessive & dominant
Genotype
An organism’s genetic makeup (GG, Gg or gg).
The genotypic ratio in Mendel’s experiment of the F2 generation was 1:2:1 (1GG:2Gg:1gg).
TESTCROSS
The breeding of an organism of unknown genotype with a homozygous recessive to determine whether an organism with a dominant phenotype (e.g. green pod color) is homozygous dominant or heterozygous
- All offspring were homozygous dominant = organism was homozygous dominant.
- all green pods since a GG x gg cross produces Gg progeny.
- Some recessive = heterozygous.
- both green and yellow phenotypes, since a Gg x gg cross produces Gg and gg progeny in a 1:1 ratio.
- The testcross was devised by Mendel and is still an important tool in genetic studies
Classic pattern w/ homozygous dominant w/homozygous recessive parents
All heterozygous first, skip generation then get 3:1 ratio.
Mendel’s principle of segregation
- Proposed that alleles segregate from one another during the formation of gametes & each reproductive cell carries only one of the homologous genes
- A phenotypic ratio of 3:1 in the offspring of a mating of two organisms heterozygous for a single trait is expected when: The alleles segregate during meiosis
- Important in horsebreeding – to know truebred
Who expresses X-linked genes?
Dominant vs recessive
Traits determined by either dominant or recessive X-Linked genes are expressed in the male.
Why can’t the genes on the X Chromosome be transmitted from father to son?
Males have only one X chromosome and don’t pass one on to their sons, who are XY
mendel’s principle of independent assortment
Hereditary transmission of one gene has no effect on the transmission of another
4 major modes of inheritance
autosomal dominant
autosomal recessive
X-linked dominant
X-linked recessive
(last two occur only on X chromosome. First 2 on 22 autosomes)
only a few disease causing genes, primarily affecting male fertility, on Y chromosome
Pedigree chart
summarizes family relationships and shows which members of a family are affected by a genetic disease
Proband (aka propositus/proposita): first person in family Dxed or seen in clinic
Females and X-linked disease
Females receive 2 X chromosomes, one from father, one from mother, so can be homozygous for a disease allele at a locus, homozygous for the normal allele, or heterozygous.
Female w/disease is very rare: An abnormal gene on the X chromosome from each parent would be required, since a female has two X chromosomes. This could occur in the two scenarios below.
- For a given birth, if the mother is a carrier and the father has the disease:
25% chance of a healthy boy
25% chance of a boy with the disease
25% chance of a carrier girl
25% chance of a girl with the disease
- If the mother has the disease and the father has the disease:
100% chance of the child having the disease, whether boy or girl.
The odds of either of these two scenarios are so low that X-linked recessive diseases are sometimes referred to as “male only” diseases. However, this is not technically correct.
Female carriers can have a normal X chromosome that is abnormally inactivated. This is called “skewed X-inactivation.” These females may have symptoms similar to those of males.
Males and X linked disease
Males have only one X chromosome = they are hemizygous
This means if they get a recessive disease gene, they will be affected. The Y chromosome does not carry the normal allele to counteract the disease causing allele (as is more possible in the female)
X-Linked disease: dominant vs recessive
Most X-Linked diseases are recessive
Sex-Linked Inheritance Question
Hemophilia is a sex-linked trait where XH gives normal blood clotting and is dominant to the hemophilia allele Xh.
Give the genotypes of:
1) a woman with normal blood clotting whose father had hemophilia and
2) a normal man whose father had hemophilia.
1) the woman has normal clotting so she has one XH but she got Xh from her father
2) the man is XHY since he got the Y from his father and he is normal so must be XH
XHh & XHY
- What is the probability that a mating between these two individuals will produce a child, regardless of sex, that has hemophilia:
- If this couple has a daughter, what is the probability that the daughter will be a carrier of the hemophilia trait?
- What is the probability a daughter would have hemophilia?
- If this couple has a son, what is the probability he will have hemophilia?
- each child has a 1/2 chance of being male and males have a 1/2 chance of being affected; so 1/4 chance of a child with hemophilia
- daughter w/ 1/2 chance of being a carrier
- 0 chance that a daughter would have hemophilia
- 1/2 chance
Phenotype & environment
Phenotype is result of both genotype & environment
PKU: untreated –> mental retardation; dietary restrictions change phenotype–> no mental retardation
Codominance
heterozygote distinguishable from both homozygotes.
AB blood type: both alleles are detectable
germline mosaicism
2+ offspring show up w/traits of autosomal dominant d/o when no family Hx
most likely: germline mosaicism - during embryonic development of one of parents, a mutation affected all or part of germline but few to no somatic cells (=in germline, but not expressed in parent)
