Genetics Flashcards
gene
determinants of a particular characteristic
- can be heterozygous or homozygous
allele
variant form of a gene
genotype
genetic constitution summing all of the alleles
phenotype
observed form of a character of an organism
Mendel’s Experiments
- proposed rules to predict types of progeny and proportions of each type knowing information about the parents
- single gene model with equal segregation of alleles
DNA during cell division
- meiosis separates homologous chromosomes
- sister chromatids separate and migrate to opposite poles (mitosis)
- meiosis gives cells with single versions of each allele
- each gamete has one version of an allele
Dihybrid Ratios
- Mendel looked at two genes segregating in the pea hybrids
- independent assortment of alleles shows each possible fertilisation event is independent of each other
Wild-type and mutant alleles
wild-type: more frequent allele
if there is no wild type allele the gene would’ve been polymorphic
Sex-linked inheritance
- females have homomorphic pair (X,X)
- males have heteromorphic pair (X,Y)
Sex linked traits more often found in males and are carried on the X chromosome
Lethal alleles
- disrupt 3:1 ratio
eg. agouti mice: - YY allele is lethal, but Yy is not
- therefore 2:1 ratio (Yy:yy) is produced indicating a lethal allele is present
Environmental influence on phenotypes
- Phenylketonuria (PKU) - rare recessive allele
homozygotes have severe mental retardation - Codes for Phenylalanine Hydroxylase
- Affected individuals can’t convert Phe to Tyr
- Phe accumulates in cerebrospinal fluid.
- Changing diet (environment) of PKU individuals at birth (to low protein) allows them to be virtually normal (wild type)
Departures from Mendelian Ratios
- if more than one gene affects the same characteristic, novel phenotypes and unusual ratios will appear
- this is still the Mendelian dihybrid ratio, but with differing phenotypes:
9 doubly dominant phenotype
3 dominant first recessive second gene
3 recessive first dominant second gene
1 doubly recessive
Genotype Masking
recessive epistasis: the expression of one gene pair is dependent on another gene pair
eg. laboradors
- recessive ee genotype (yellow) masks expression of any other genotypes (B)
Gene Linkage + Crossing Over
- genes on the same chromosome are usually inherited together
- causes differing allele ratios
- crossing over is the breaking and rejoining of homologous chromosomes in the same position to generate recombinants
Genetic Maps
- The phenomenon of recombination or genetic exchange allows for the construction of genetic maps
- Genes are placed on the map as a result of linkage to other genes whose position is already known
- The map distance is the unit of recombination - 1% of recombination frequency or 1cM