Inheritance

Phenon

the sum of all traits expressed by an
individual

Law of Segregation

MA and PA
chromosomes/alleles segregate into
different daughter cells during Meiosis I

This is why after Meiosis 2

• gametes lack paired, homologous
chromosomes (homologues)
• gametes have half the total number of
chromosomes as germ cells (2n ➟ n)

Without chromosome reduction during
meiosis, chromosome number would
double following syngamy during every
generation

nuclear/cellular instability

Law of Independent Assortment

alleles at different gene loci assort into gametes
independently due to events that follow
homologue synapsis/tetrad formation

Prophase I

replicated chromatin
condenses, synapses, and alleles cross-
over among synapsed non-sister
chromatids (red & blue) at chiasmata ➟
genetic recombination

hybrid chromosomes

have new
combinations of MA and PA alleles that
may have never existed before

Metaphase I

each tetrad connects to
centrosomes via independent spindle
fibers resulting in random, independent
orientation in relation to cell poles

Dominant
allele

Always expressed;
represented by capitol
letter (e.g., R).

Recessive
allele

Only expressed in
absence of dominant
allele; represented by
lowercase letter (e.g., r)

Genotype

The alleles an individual
inherits for a gene locus
(e.g., RR, Rr, or rr)

Phenotype

The expression of a gene
locus as a trait (e.g., Rh+
or Rh– blood

Homo-
zygous

Same alleles at a gene
locus (e.g., RR or rr)

Hetero-
zygous

Different alleles at a gene
locus (Rr)

Mendelian trait

is a trait controlled by a single gene
locus expressed as only a few (often dichotomous),
discrete (non-overlapping) character states (e.g., Rh+
vs Rh– blood; A, B, AB, or O Blood Type)

Pleiotropic gene

a single gene locus whose
expression impacts multiple traits (e.g. SCD patients
likely have compromised thermoregulation, immune
responses, and decreased response to vaccines)

Rh factor gene locus is on homologue 1 and its
expression is governed by complete dominance:

only
dominate allele (R) is expressed in heterozygotes (Rr)
Rh factor: Complete Dominance

ABO gene locus is on homologue 9 and its
expression is governed by co-dominance:

similar to complete dominance but...
• two dominant alleles (IA, IB) in the population
results in two heterozygous genotypes (IA i, I B i)
and one co-dominant (IA IB)
ABO Blood Type: Co-Dominance

β-globin locus is on homologue 11 and its expression
is governed by incomplete dominance:

heterozygote
has expression intermediate that of either
homozygote

FVIII gene locus is on the X sex chromosome
and its expression is governed by X-linked
inheritance:

expression depends on number of
X sex chromosomes (♀=XX; ♂=XY; ⚥=varies)

Recessive alleles

usually result from mutations that
prevent an individual from making a protein

Dominant alleles

may not be the most frequent in a
population (e.g., ABO Blood Type) due to selection

transposons

DNA sequences that insert in
nearby gene loci disrupting transcription

epigenetic modifications

chromatin is
chemically modified (e.g., methylated, Me, –CH4)
disrupting ‘unpacking’ necessary for transcription