Chapter 12

Allele: alternative forms of a gene, found at the same place on a chromosome that codes for a specific genetic trait (genetic makeup given from parents)

Dominant: alleles are expressed when present

Recessive- alleles are only expressed when there is no dominant allele present

Genotype- the genetic combination of alleles/allele makeup of an individual

Phenotype (trait)- the observed or measurable physical characteristics we see in an individual

Homozygous: two alleles that are the same

• Homozygous dominant: YY

• Homozygous recessive: yy

Heterozygous: two different alleles

• Heterozygous: Yy

Punnett square: all possible allele combinations can be predicted

• It ensures that you consider all possible combinations of gametes when calculating expected genotype frequencies

Mendel's experiments with pea plants led to the discovery of how traits are passed down through generations, using the scientific method

• Mendel used two types of fertilization in his experiments using either 1 or 2 parent organisms

Self-Fertilization- a reproductive process where the male and female gametes (sex cells) of the same organism fuse (apply pollen from a male organ to a female organ on the same plant)

True Breeding: homozygous self-fertilization produces offspring with the same phenotype as the parent

Hybrid: heterozygous self-fertilization produces offspring with mixed phenotype

Cross-fertilization: Mendel discovered dominant and recessive traits when he cross-fertilized yellow and green pea plants

• When analyzing offspring Mendel always found yellow peas (dominant trait)

• Sometimes Mendel found a mixture of phenotype by Punnet Square but never all green offspring (green pea plants = recessive)

• This indicated that some yellow pea plants are heterozygous

Mendel’s First Law- Law of Segregation: the two copies of a gene separate during gamete formation; each gamete receives only one copy

• During gamete formation, 2 alleles of the same gene separate and end up in different gametes → gametes are haploid and only receive one copy of a gene/allele

• More applicable in monohybrid crosses

• ex: Gametes from a parent with the RR genotype will all carry the R allele; gametes from an rr parent will all carry the r allele. In an Rr individual, half the gametes will carry the R allele and the other half will carry r

Mendel’s Second Law- Independent Assortment: copies of different genes assort independently → the inheritance of one trait does not influence the inheritance of another trait

• Mendel mentioned that the inheritance of multiple genes to discover dihybrid crosses

• Independent assortment is more applicable in dihybrid crosses

Mendel standardized the naming system of generation to track inheritance patterns:

• P generation– Parental Generation: the original (parent) set of individual plants that mated

• F1 generation – First filial generation: offspring of the P generation (children)

• F2 generation – Second filial generation: offspring of the F1 generation

Monohybrid Crosses: a breeding experiment between two heterozygous organisms that have different variations of a single gene (cross parental varieties with contrasting traits for a single character/trait)

• F₁ are monohybrids

• Allow for plants to self-pollinate to produce F₂ generation

• Ex: Mendel’s monohybrid cross that he conducted: flower color (purple vs. white), seed shape (round vs. wrinkled), and plant height (tall vs. dwarf)

Phenotypes do NOT always reveal the genotype (ex: dominant yellow pea could be YY or Yy)

Dihybrid Crosses: a breeding experiment that involves crossing two organisms that are both heterozygous for two different traits WATCH A VIDEO ON HOW TO DO A DIHYBRID AND MONOHYBRID CROSS ON A PUNNET SQUARE AND PRACTICE

• Mendel’s test: cross of peas differing in two characterisitics, seed shape, and color

• Seed shape and color: Round, Yellow seeds (RRYY) and Wrinkled, Green seeds (rryy)

• F1, generation is RrYy – all round and yellow

• F2 would have 9 different genotypes; phenotypes would be in 9:3:3:1 ratio

Pedigrees can be used to determine whether a rare allele is dominant or recessive

• females: circles

• Males: squares

• Shaded circle or square: affected male or female

• Unshaded circle or square: unaffected male or female

Autosomal dominant: a pattern of inheritance for genetic traits or disorders where a mutated gene from one parent is enough to cause the condition in a child

• The gene is located on a non-sex chromosome, also known as an autosome

Autosomal recessive: a pattern of inheritance for genetic traits or disorders that occurs when a person needs to inherit two copies of a mutated gene, one from each parent, in order to receive that trait/disorder

X-linked recessive: a pattern of inheritance for genetic conditions that are caused by mutations in the X chromosome

• Males only have one X chromosome, so a mutation in that gene will cause the condition to appear

• Females have two X chromosomes, so a mutation in both X chromosomes is required for the condition to appear. If a female has one mutated X chromosome and one normal X chromosome, she is a carrier and will not be affected by the condition

Color blindness is a sex-linked recessive trait

Incomplete dominance: Alleles are neither dominant nor recessive – heterozygotes have intermediate phenotypes

• ex: a purple fruit (PP) is bred with a white fruit (pp), and the offspring are fruit with a violet color (Pp)

Codominance: Alleles produce phenotypes that are both present in the heterozygote (not a blend, just includes both traits that you can see)

• ex: People in the AB group make both A and B antigens, and neither antibody. The I^A and I^B alleles are codominant

Pleiotropic: one allele has multiple phenotypic effects

• ex 1: Phenylketonuria results from a mutation in the gene for a liver enzyme that converts phenylalanine to tyrosine. Phenylalanine builds up toxic levels, which affects development in many ways

• ex 2: Albinism: a disease that causes a lack of melanin in hair, skin, and eyes

• ex 3: Malaria: someone without sickle cell disease is susceptible to malaria, but someone producing sickled cells is immune to malaria

Epistasis: the action of when one gene alters and masks the phenotypic effect of another gene

• ex: Coat color in Labrador retrievers- Alleles for black and brown aren’t expressed unless alleles E (for pigment deposition) are expressed. An ee dog is yellow regardless of which B alleles are present. E is epistatic to B (E "overrides" B).