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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
What discovery did Mendel's experiments lead to?
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 and cross fertilization)
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 pure yellow and pure green pea plants
When analyzing the F1 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)
Since some of the F1 yellow pea plants produced both yellow and green peas in the next generation (F2), Mendel concluded that these yellow pea plants must have been heterozygous after the first cross
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
Law of 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)
TRUE OR FALSE: Phenotypes always reveal the genotype
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
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 IA and IB 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).
