1. Mendelian Patterns of Inheritance
BIOLOGY
Mendelian Patterns of Inheritance

2. Gregor Mendel ( )
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.

3. Gregor Mendel—Important Dates
Experimented with peas, Pisum sativum,
from
Reported his findings to the Brunn Society for the Study of Natural Science in 1865
Published Experiments with Plant Hybrids in 1866

4. 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

5. 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.

6. Genotype Versus Phenotype
Refers to the alleles an individual receives.
Phenotype
Refers to the physical appearance of the individual.

7. 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

8. 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.

9. 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)

10. Two-Traits Dihybrid 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.

11. Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

12. 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.

13. Incomplete Dominance
Exhibited when the heterozygote has an intermediate phenotype between that of either homozygote.

14. Co-Dominance
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

15. 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

16. 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)

17. 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

18. Environment and Phenotype
Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.
Black fur only occurs on extremities.