Biology: Heredity

Sexual Reproduction

Requires male and female

Asexual Reproduction

One parent; example: binary fission

Genome

All of an organism's DNA

Gene

Codes for a specific trait

Locus

Location of a gene on a chromosome

Allele

• Variation  (2 per chromosome 1 per parent Aa)

  • More than 2 alleles possible for some traits (e.g., blood type codominant)

  • O is recessive when crossing blood types 

Wild Type

Most common allele in a population

Mutant Allele

Has a change in genetic code

Genotype

Genetic makeup (e.g., AA, Aa)

Phenotype

Observable traits (e.g., hair color, height)

Homologous Chromosomes

One from each parent

Hemizygous

Only one allele present from sex (e.g., males with X-linked genes)

Penetrance

How often it shows up (all have trait complete half incomplete)

Expressivity

How bright a color is

Complete Dominance

One allele completely masks the other

Incomplete Dominance

Blended traits (red + white = pink)

Codominance

Both alleles are fully expressed (e.g., AB blood type)

Epistasis

•  One gene affects the expression of another

  • Example: Baldness gene overrides red hair gene

Polygenic Inheritance

Multiple genes affect one trait (e.g., height)

Pleiotropy

One gene affects multiple traits

Haplosufficiency

One functional gene is enough

Haploinsufficiency

One gene is not enough for normal function

Proto-oncogenes

• Normal genes promoting cell growth (haploinsufficient)

  • Gain-of-function mutation: Leads to oncogene (cancer-causing)

  • One-Hit Hypothesis: Only one mutation needed

Tumor Suppressor Genes

• Inhibit cell growth (haplosufficient)

  • Loss-of-function mutation: Gene stops working

  • Two-Hit Hypothesis: Both copies must be mutated

Examples of Tumor Suppressor Genes

P53, P21, RB (Retinoblastoma)

P53

Responds to stress

P21

Decreases cell division (inhibit cyclin)

RB (Retinoblastoma)

Controls interphase growth

Null Allele

Has no function

Mendelian Laws (Gregor Mendel)

• Law of Dominance: One allele can mask another

• Law of Segregation: Alleles separate in Anaphase I (homologous)

• Law of Independent Assortment: Chromosomes line up independently in Metaphase I

  • 2²³ possibilities

Crossing Over

Exchange of genes in Prophase I

Where is the site of crossing over?

Chiasma

Nondisjunction

• Improper segregation of chromosomes during anaphase

  • Meiosis I: All gametes affected (2 n+1, 2 n-1) (22 , 24)

  • Meiosis II: Half normal, half abnormal

  • Mitosis: Same result as Meiosis II

Aneuploidy

• Abnormal chromosome number

  • Monosomy (1), Trisomy (3), Disomy (2 - normal)

Recombination

Result of crossing over

Recombination Frequency

• Depends on distance between genes 

• Farther = more recombination

  • < 50% = linked

  • 50% = unlinked (never greater than .5)

X-Linked Dominant

Affects all with the X

X-Linked Recessive

• Females: Need 2 copies to show trait (1 = carrier)

• Males: Only 1 X = affected

• Examples: hemophilia and colorblindness

Y-Linked

Passed from father to son

Genomic Imprinting

Genes expressed based on external factors

Epigenetic Changes

• Affect gene expression, not DNA sequence

  • DNA Methylation, Histone Acetylation, Histone Deacetylation, Histone Methylation

DNA Methylation

Suppresses gene expression

Histone Acetylation

Loosens DNA, increases transcription

Histone Deacetylation

Tightens DNA, decreases transcription

Histone Methylation

Effect depends on context

Autosomal Traits

Traits not skip generations and affect both males and females

Sex-Linked Traits

Unequal distribution between sexes

Dominant

Seen in every generation

Recessive

Can skip generations

To determine the number of unique gametes, use the following formula:

number of unique gametes = 2ⁿ

• n is the number of heterozygous gene pairs.