Mendelian Genetics

(not included) hybrid crosses, punnett squares, pedigrees

Mendel’s Significance

performed breeding experiments between reciprocal pea plants to track inheritance of contrasting traits in offspring.

Heterozygous Cross Ratios

3:1 in dominant : recessive phenotypes & 1:3:1 in DD:Dd:dd genotypes

Phenotype

observable characteristics of an organism based on its’ genotype’s interaction with environment.

Homozygous

an organism with 2 identical alleles (dominant or recessive e.g. DD or dd).

Heterozygous

an organism with 2 different alleles (1 dominant & 1 recessive e.g. Dd) which will always be phenotypically overruled by the dominant allele.

Mendel’s 4 Main Postulates/Laws

1. unit factors in pairs (gene controlling a trait, given in pairs, always results in DD, Dd or dd)

2. dominance/recessive (within 2 different alleles, there’s a dominant one that overrules)

3. segregation (meiosis ensures each gamete receives only 1 gene allele - 50/50 thing)

4. independent assortment (allele for one thing doesn’t affect an allele for another thing)

(n!/s! x t!) x a^sb^t

binomial expansion used to calculate offspring probabilities’ geno/pheno types.

n = number of trials (offsprings etc) / s = number of times a happens / t = number of times b happens

where s+t = n

Epistasis

gene interaction where one gene’s effects masks/modifies the effects of another gene (at a different locus, therefore unrelated) leading to a difference in expected phenotype. (e.g. having the gene for black hair, but also having the gene for baldness)

Recessive & Dominant Epistasis

genetic phenomenon where the epistatic gene is on the recessive (for r-epistasis ratio → 9:3:4) or dominant (for d-epistasis ratio → 12:3:1) allele, masking the other (unrelated) allele regardless if it’s meant to code for a phenotype. the difference is the dominant only needs 1 allele to work while the recessive needs 2 alleles.

Duplicate Recessive/Dominant Epistasis

recessive = recessive allele of 2 genes masks the dominant alleles (leads to a 9:7 ratio)

dominant = dominant allele of 2 genes masks the recessive alleles (leads to a 15:1 ratio)

Novel Phenotype

new/unique trait/appearance in offspring due to (similar) genes interactions which are completely different to parental phenotypes (i.e. incomplete dominance).

Incomplete Dominance

a blended “novel” phenotype (red x white = pink), a uniform mix of the parental phenotypes.

Haploinsufficiency

phenomenon where having only 1 functioning copy of a gene is not enough to produce a necessary protein or gene product for normal function, resulting in an altered phenotype.

Co-dominance

two different alleles are expressed simultaneously, resulting in a phenotype that expresses both alleles (red + white = red & white or AB blood type).

Lethals

(rarely dominant and commonly recessive) alleles that will cause the death of an organism. (e.g. huntington’s disease, homozygous = embryonically lethal, heterozygous = later in life)

*recessive lethals can appear dominant due to one copy of its allele changes the organisms appearance, however, two copies can result in death.

X-Linkage/Inheritance

a gene located on the X chromosome (23rd pair).

*in a male, X carries more genes than the Y chromosome, therefore lacks a partner gene = one allele for that gene (hemizygous). should parents swap x-linked trait = different outcomes.

**overall: affected mothers → affected sons & affected fathers → carrier daughters

2 Main Factors that Affect an Organism’s Phenotype

Sex (autosomal gene = prominent in one gender than the other) & Environment (e.g. temp/diet → changes phenotype).

Penetrance

percentage of individuals that show the phenotype (e.g. 80/100 people show symptoms = 80% penetrant).

Expressivity

due to penetrance, measures range of trait’s strength in expression (e.g. polydactyly → some have full extra finger, some have a nub).