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What is genetics?

The study of how genes and DNA control traits, how those traits are inherited, and how they vary in populations

What is a gene?

A specific sequence of DNA that contains instructions for making a functional product (usually a protein), including regulatory regions and coding regions

What does DNA store?

DNA stores genetic information as sequences of bases (A, T, G, C) that determine which proteins are made and how cells function

Central dogma

The flow of genetic information: DNA is transcribed into RNA, which is translated into protein that affects traits

How do genes affect traits?

Genes are used to make proteins, and those proteins determine structure, function, and observable traits (phenotype)

How are genes passed on?

Genes are replicated during cell division and passed to offspring through gametes (sperm/egg)

Diploid vs haploid

Diploid cells have 2 copies of each chromosome (one from each parent); haploid cells have 1 copy (gametes)

What did Mendel discover?

Traits are inherited as discrete units (genes), and each individual has two alleles for each gene

Dominant vs recessive

A dominant allele shows its effect even if only one copy is present; a recessive allele only shows if both copies are recessive

Law of segregation

The two alleles for a gene separate during meiosis so each gamete gets one allele

Law of independent assortment

Alleles of different genes are inherited independently of each other (if genes are unlinked)

Homozygous

An individual has two identical alleles for a gene (AA or aa)

Heterozygous

An individual has two different alleles for a gene (Aa)

What is meiosis?

A specialized cell division that produces haploid gametes and generates genetic variation

Key meiosis steps

Prophase I: crossing over; Meiosis I: homologous chromosomes separate; Meiosis II: sister chromatids separate

What is a mutation?

A permanent change in the DNA sequence that can affect gene function

Mutation types

Null = no function, leaky = reduced function, silent = no noticeable change in function

What is an SNP?

A single nucleotide change in DNA that can sometimes affect gene function or trait variation

Drivers of evolution

Changes in allele frequencies caused by environmental pressures, disease, and diet

Codominance

Both alleles are fully expressed in the phenotype at the same time (ex: blood type AB)

Haplosufficient

One functional allele provides enough product for a normal phenotype

Haploinsufficient

One allele is not enough, so the phenotype is affected even in heterozygotes

Autosomes vs sex chromosomes

Autosomes contain most genes; sex chromosomes (X and Y) determine biological sex and carry some unique genes

Female gametes

All eggs carry an X chromosome because females are XX

Male gametes

Sperm carry either X or Y chromosomes, determining offspring sex

What determines sex?

The presence of the SRY gene on the Y chromosome triggers male development

Sex-linked traits

Traits controlled by genes located on sex chromosomes

X-linked traits

Genes on the X chromosome; often affect males more because they only have one copy

Y-linked traits

Genes on the Y chromosome; only males have and pass them

Why males often affected (X-linked)?

Males have only one X, so any mutation will be expressed without a second copy to mask it

Carrier female

A heterozygous female who has a recessive mutation but does not show the phenotype

Pedigree symbols

Squares = males, circles = females, shaded = affected individuals

Autosomal recessive

Requires two mutant alleles; can skip generations because carriers are unaffected

Autosomal dominant

Requires only one mutant allele; usually appears every generation

X-linked recessive

More males affected; females need two copies; affected daughters must have affected fathers

X-linked dominant

All daughters of an affected father are affected; sons are not

Y-linked pattern

Trait appears only in males and is passed from father to son

Hemophilia inheritance

X-linked recessive disorder affecting blood clotting

What is a dihybrid cross?

A genetic cross involving two different genes/traits at the same time

Dihybrid ratio (independent)

9:3:3:1 phenotypic ratio resulting from two independently assorting genes

What does 9:3:3:1 indicate?

The parents were heterozygous for both genes and the genes are unlinked

Law of independent assortment

Alleles for different traits separate independently into gametes (if not linked)

When does independent assortment fail?

When genes are located close together on the same chromosome (linked)

Product rule

Used when calculating probability of multiple independent events occurring together (multiply probabilities)

Sum rule

Used when calculating probability of mutually exclusive outcomes (add probabilities)

Branch diagram

A visual method to track probability outcomes step-by-step for multiple traits

Linked genes

Genes located on the same chromosome that tend to be inherited together

Recombination

Crossing over during meiosis that creates new combinations of alleles

When does recombination occur?

During prophase I when homologous chromosomes exchange segments

Mitochondrial DNA inheritance

Mitochondrial DNA is inherited only from the mother because the egg provides cytoplasm

Mito inheritance pattern

All children of an affected mother inherit the trait, but only daughters pass it on

What is a locus?

The specific physical location of a gene on a chromosome

Purpose of gene mapping

To determine gene order and distance based on recombination frequency

Linkage evidence

When parental phenotypes are more common than recombinant phenotypes

Parental types

Offspring with allele combinations identical to the parents

Recombinant types

Offspring with new allele combinations created by crossing over

Cis configuration

Both dominant (or both recessive) alleles are on the same chromosome (AB/ab)

Trans configuration

Dominant and recessive alleles are mixed between chromosomes (Ab/aB)

Recombination frequency (RF)

The percentage of offspring that are recombinant; used to measure gene distance

RF meaning

Higher RF means genes are farther apart; lower RF means they are closer together

1 map unit

Equal to 1% recombination frequency

Gene distance effect

The farther apart genes are, the more likely crossing over occurs

Mapping multiple genes

Compare recombination frequencies between gene pairs to determine order

Multiple alleles

A gene can have more than two forms in a population, even though individuals only carry two

Pleiotropy

A single gene affects multiple, different traits in an organism

Recessive lethal alleles

Alleles that cause death when homozygous, leading to a 2:1 survival ratio instead of 3:1

Complete dominance

One allele completely masks the effect of another in heterozygotes

Codominance

Both alleles contribute fully to the phenotype (no masking)

Incomplete dominance

The heterozygote shows an intermediate phenotype between the two alleles

Gene interaction

Multiple genes interact in pathways to influence a single phenotype

Pathway concept

Traits are produced through multi-step biochemical pathways; disruption anywhere affects outcome

Complementation test

A cross between two mutants to determine if mutations are in the same or different genes

WT offspring (complementation)

Mutations are in different genes; each parent provides a functional allele

Mutant offspring (no complementation)

Mutations are in the same gene; no functional allele is present

Allelic mutations

Mutations occur in the same gene (no complementation)

Non-allelic mutations

Mutations occur in different genes (complementation occurs)

9:7 ratio

Complementary gene interaction where both genes are required for a phenotype

9:3:4 ratio

Recessive epistasis where one gene can mask another when homozygous recessive

12:3:1 ratio

Dominant epistasis where a dominant allele masks another gene’s effect