Mendelian Patterns of Inheritance

BIOLOGY

Gregor Mendel (1822-1884)• Austrian monk who formulated

fundamental laws of heredity in early 1860s. – Studied science and

mathematics at University of Vienna.

– Formulated the particulate theory of inheritance.

• Inheritance involves reshuffling of genes from generation to generation.

Gregor Mendel—Important Dates• Experimented with peas,

Pisum sativum,

from 1857-1865

• Reported his findings to the Brunn Society for the Study of Natural Science in 1865

• Published Experiments with Plant Hybrids in 1866

One-Trait Inheritance• Mendel performed cross-breeding

experiments between true-breeding plants (homozygous).– Chose varieties that differed in only one

trait (monohybrid cross)– Performed crosses

• Parental generation = P• First generation offspring = F1 (1st filial)• Second generation offspring = F2

Law of Segregation• Each individual has two

factors for each trait.

• The factors separate when gametes form.

• A gamete contains only one of two factors.

• Fertilization = new individual with 2 factors for each trait.

Genotype Versus Phenotype• Genotype

– Refers to the alleles an individual receives.

• Phenotype – Refers to the

physical appearance of the individual.

Modern Genetics View• Each trait is controlled by two

alleles (alternate forms of a gene).• Dominant allele (D) masks the

expression of the recessive allele (d).

• Alleles occur on a homologous pair of chromosomes at a locus.– Homozygous = identical alleles– Heterozygous = different alleles

Punnett Square• All possible kinds of

sperm are lined up on one axis, and all possible kinds of eggs are lined up on the other axis.– Every possible

combination of alleles is placed within the squares.

Testcross• A monohybrid testcross determines if the phenotype is homozygous dominant (RR) or heterozygous (Rr).

• Those with one dominant gene will express the dominant phenotype whereas individuals with the recessive phenotype must be homozygous (rr)

Two-TraitsDihybrid Cross• Mendel performed cross

using true-breeding plants differing in two traits.

– Observed phenotypes among F2 plants.

– Formulated law of independent assortment:• Each factor segregates

independently of the others• All possible combinations

of factors can occur.

Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

Two-Trait Testcross

• A two-trait testcross is used to determine if an individual is homozygous dominant or heterozygous for either of the two traits.

Incomplete Dominance• Exhibited when the heterozygote has an

intermediate phenotype between that of either homozygote.

• Most genes have more than two alleles in a population—called multiple alleles.

• The ABO blood groups in humans are determined by three alleles, IA, IB, and i.– Both the IA and IB alleles are dominant to the i

allele – The IA and IB alleles are codominant to each

other.

• Because each individual carries two alleles, there are six possible genotypes and four possible blood types.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Co-Dominance

Polygenic Inheritance• Occurs when a trait is governed by two or

more sets of alleles.– Each dominant allele has a quantitative effect

and these effects are additive like height

Terminology• Pleiotropy - A gene that affects more than one characteristic of an individual.

• Sickle-cell (incomplete dominance)

• Epistasis - A gene at one locus interferes with the expression of a gene at a different locus.– Human skin color (polygenic

inheritance)

Example of Epistasis– In mice, coat color depends on

two genes.– The epistatic gene determines if

pigment will be deposited in hair or not.• Presence (C) is dominant to absence

(c).

– The second determines what pigment is deposited: black (B) or brown (b).• The black allele is dominant to the

brown allele.

– An individual that is cc has a white (albino) coat regardless of the genotype of the second gene.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Environment and Phenotype

• Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.– Black fur only occurs on extremities.