Bio: Genetics and Evolution
each pair of chromosomes consists of homologous chromosomes, which means that they have the same genes in the same locations
a gene is a specific segment of DNA that codes for a protein or functional RNA
linked genes are located close together on the same chromosome and tend to be inherited together
unlinked genes are located on different chromosomes or are far apart on the same chromosome, meaning they assort independently
Mendel’s first law reveals that some alleles are dominant and can mask the expression of recessive alleles in the phenotype
Mendel’s second law states that each individual has two alleles for each trait, one from each parent.
Mendel’s third law (the law of independent assortment) suggests that genes for different traits are inherited independently of each other.
the inheritance of an allele for one trait does not affect the inheritance of an allel for another
pedigrees:
circle is female
square is male
unshaded means unaffected, whereas shaded means affected
half-shaded represents carriers
a horizontal line connecting a male and a female represents mating
vertical lines extending downward indicate offspring
a diagonal line through a symbol indicates a deceased individual
autosomal dominance inheritance: just one dominant allele can cause a trait or disorder
autosomal recessive inheritance: requires that an individual receive 2 recessive alleles, one from each parent, to express the trait or disorder
X-linked inheritance: since males only have one X chromosome, they express all X-linked traits, even if recessive, because they lack the 2nd X to mask mutations
Females can be homozygous or heterozygous for X-linked traits, so they could be carriers of recessive disorders without expressing symptoms
fathers pass X chromosome to daughter, never to son
Y-linked inheritance:
very few genes on the Y chromosome
Y-linked traits are passed exclusively from father to son
SRY gene controls male development by initiating testis formation
SRY gene on the Y chromosome indicates male development
sex is determined by the presence or absence of the Y chromosome
a female with two copies of a recessive X allele will display the associated phenotype, but if she only has one copy, then she is a carrier
can still pass off to offspring
incomplete dominance: occurs when the heterozygous phenotype if an intermediate between both homozygous phenotypes
neither allele is completely dominant, and the resulting phenotype resembles a mix between the two parent traits
codominance: both alleles in a heterozygote are fully expressed without blending
multiple allele inheritance: a gene has more than two possible alleles within a population, though any individual still inherits only two alleles
while a single organism can only have two copies of the gene, the population has more thantwo possible versions of the allele cirulating
ex: ABO blood type inheritance, controlled by the I gene with three different alleles:
IA
IB
IO
IA and IB are codominant, so people with both would be type AB
mitochondrial DNA is inherited exclusively from the mother because the mitochondria in sperm are typically destroyed after fertilization
an affected father cannot pass the mutation down to his children since it is only passed down from mother to child
epistasis: where one gene masks or modifies the expression of another gene at a different locus
polygenic inheritance: where multiple genes contribute to a trait, resulting in continuous variation
linked genes: where two or more genes are located close together on the same chromosome and tend to be inherited together
pleitropy: describes a single gene influencing multiple, often seemingly unrelated phenotypic traits
penetrance: the proportion of individuals with a specific genotype who actually express the expected phenotype
how consistent a gene is at showing up as a trait
complete: if a gene has 100% penetrance
expressivity: the degree to which a genotype is expressed in an individual
about variation in the severity or presentation of a trait
since females have two X chromosomes, they can have an excess in gene expression. to prevent this, cells in female mammals undergo X-inactivation, a process that silences one of the two X chromosomes in each cell
nondisjunction: is an error in meiosis or mitosis where chromosomes fail to separate correctly, leading to abnormal chromosome numbers in daughter cells
major cause of aneuploidy, which can result in genetic disorders such as Down syndrome (trisomy 21) or Turner syndrome (monosomy X)
can happen at 2 stages of meiosis:
meiosis 1 - fail to separate at anaphase I, resulting in 2 gametes with an extra chromosome and two gametes missing a chromosome
meiosis ii - fail to separate during anaphase II. results in two normal gametes, one with an extra chromosome, and one with a missing chromosome
less severe than meiosis I
aneuploidy: a gain or loss of individual chromosomes
polyploidy: a gain of entire extra sets of chromosomes
chromosomal mosaicism: a individual has two or more genetically distinct cell populations
point mutations: involve changes in a single nucleotide base in the DNA sequence
three main types
insertion occurs when one or more nucleotides are added to the DNA sequence, potentially disrupting the entire gene
frameshift insertion shifts the reading frame, altering all downstream amino acids
nonframeshift insertion occurs in multiples of three, preserving the reading frame
deletion mutations occur when one or more nucleotides are removed from the DNA sequence
also cause frameshift or nonframeshift deletion
substitution mutation occurs when one nucleotide is replaced by another, which may or may not affect protein function
less disruptive than insertions, unless they introduce a stop codon
if the change in the DNA sequence does not alter the protein’s amino acid sequence, it results in a silent mutation
if the substitution changes the codon to one that specifies a different amino acid, it is a missense mutation
if the substitution changes the codon to a stop codon, it is a nonsense mutation