Inheritance

codominant gene

two alleles are expressed simultaneously in an individual

dominant gene

only this allele is expressed

incomplete dominance

an intermediate phenotype is expressed

recessive gene

this allele is not expressed

law of segregation

two alleles for a gene segregate during gamete formation and end up in different gametes

monohybrids

heterozygotes for one particular characteristic

dihybrids

heterozygotes for two characteristics

pleiotropy

one gene affects multiple phenotypic characters

polygeny

two genes affect one phenotypic character

epistasis

one gene locus masks or modifies the phenotype of another gene locus

sex-linked gene

a gene located on a sex chromosome

hemizygotes

males with one locus of an allele

conditions related to x lined genes

Duchenne muscular distrophy, haemophilia, colour blindness

linked genes

genes on the same chromosome that tend to be inherited together, rarely separate during meiosis

what causes the separation/recombination of linked genes?

crossing over

parental types

offspring with the phenotypes of the parents

recombinant types

offspring with a new combination of phenotypes

ratio of parental and recombinant types

50/50

assumption of linkage maps

the farther apart two genes are, the higher their probability of crossing over

aneuploidy

an abnormal number of chromosomes due to errors in meiosis

monosomic

one less chromosome, 2n-1

trisomic

one more chromosome, 2n+1

polyploidy

more than two complete sets of chromosomes

triploidy

3n

tetraploidy

4n

alterations in chromosome structure

deletion/duplication of a segment, inversion of a chromatid, translocation

genomic imprinting

process where a gene is expressed from only one parent while the other parent's copy is silenced, vital for embryonic development

mechanism of genome imprinting

methylation of cytosine in the allele that is silenced

Independent traits (Independent assortment) scenario

• genes are on different chromosomes

• Test cross ratio = 1 : 1 : 1 : 1 (25% AB, 25% Ab, 25% aB, 25% ab)

• follows Mendel's Law of Independent Assortment: Alleles separate completely randomly.

• independent assortment happens in metaphase 1 and anaphase 1 of meiosis

Associated (Linked Genes) Scenario

• genes are on the same chromosome (closer together)

• Test cross ratio = 1 : 1 (50% Ab, 50% ab, 0% of the others)

• No crossing over happens at all. Only parental combinations are produced

Incomplete Linkage Scenario

• genes are on the same chromosome (further apart)

• Test cross ratio = unequal split (40% AB, 40% ab, 10% Ab, 10% aB)

• linkage with crossing over, parental combinations are highly frequent, recombinants are rare

• crossing over occurs in Meiosis Prophase 1