genetic inheritance

trait

characteristics of an organism

e.g. eye colour

phenotype

appearance of an organism

e.g. green eyes

genotype

genetic composition of the individual

e.g. YY, Yy, yy

gene

region of DNA influencing a trait

e.g. gene for eye colour

allele

alternative version of the same gene

e.g. Y or y

locus

specific place on a chromosome occupied by a gene

homozygous

an organism possessing 2 of the same alleles at a locus

heterozygous

an organism possessing 2 different alleles at a locus

variant

refers to a variation in genetics

• ranked on a scale

how are genetic conditions “on a scale”

mendelian (monogenic) - affects only 1 gene

polygenic - affects multiple genes

what are the 3 laws of inheritance

• principle of segregation

• principle of uniformity (dominance)

• principle of independent assortment

what is the principle of segregation

during the formation of gametes, genes segregate randomly so that each gamete receives one or the other with equal likelihood

what is the difference between predicted genotypes & phenotypes

For example:

• genotypes = RR : Rr : rr

• phenotype = rounded : wrinkled

what is the principle of uniformity

when 2 genes responsible for a single character are present, one is dominant to the other, which is recessive

what is a testcross

a test to determine if individuals are homozygous or heterozygous for a trait

• dominant phenotype & homozygous recessive

what is the principle of independent assortment

each pair of alleles segregates independently of other alleles during gamete formation

what is the difference between autosomal & sex-linked

forms of inheritance based on location

• sex-linked = X or Y chromosomes

• autosomal = other 22 chromosomes

what are carriers

individuals who are heterozygous for a variant

what is a pedigree

a representation of a family tree that outline inheritance of characteristics

what is a proband

a affected individual who first brings a family’s genetic condition to the attention of a professional

what is a consultand

an individual seeking genetic evaluation

• may be unaffected

• commonly also the proband

what are some symbols for pedigrees

what is autosomal recessive inheritance

expressed only in homozygous

• both genes mutated to express disease

• parents are usually asymptomatic carriers

• can skip generations

• ↑incidence of parental consanguinity

• subsequent children = 25% of being affected

what is consanguinity

refers to a mating couple being related

• increases chance of couple both carrying a disease-causing variant

• more often for rare diseases in the family

what is autosomal dominant inheritance

an affected person usually has at least 1 affected parent

• affects either sex

• usually doesn’t skip generations

• 1 affected & 1 unaffected parent = 50% of being affected

NOTE: affected = heterozygous

what is a reciprocal cross

a pair of crosses between a male of one strain and a female of another, and vice versa.

e.g. male = blue eyes & female = green eyes

male = green eyes & female = blue eyes

what is hemizygous

having only 1 copy of a gene rather than 2 copies

e.g. XY is hemizygous

what is some nomenclature for genetics

• {letter} + = wildtype (most common allele in natural population)

• {letter} = variant

• {capital letter} = dominant

• {lower case letter} = recessive

X-linked inheritance representation

what is some information about X-linked recessive inheritance

• affects mainly males

• usually born to unaffected parents

• mother is normally asymptomatic carrier

• can occur due to a non-random X-inactivation

• no male-to-male transmission

what is some information about X-linked dominant inheritance

• affects either sex (more females affected)

• at least 1 parent affected

• females are often more mildly affected (X inactivation)

• child of affected female has 50% chance of affected

• affected male can transmit to daughters not sons

• usually rare diseases (new mutations or germ-line mutations)

what is X chromosome inactivation

in females (XX), one X chromosome in each cell is inactivated.

• mosaics

ensures females will produce X-linked gene products in similar quantities to males → dosage compensation

what is the sum rule of probability

the probability of either of 2 mutually excusive events occurring is the sum of their individual probabilities

e.g. Pr(A or B) = Pr(A) + Pr(B)

what is the product rule of probability

the probability of independent events occurring together is the product of the probabilities of the individual events

e.g. Pr(A and B) = Pr(A) x Pr(B)