1. Patterns of Inheritance
Chapter 12

2. Learning outcomes Explain the importance of Mendel’s experiments.
Evaluate a monohybrid cross and Mendel’s principle of segregation.
Evaluate a dihybrid cross and Mendel’s principle of independent assortment.
Explain the genetic basis for continuous variation.

3. Mystery of heredity
Before the 20th century, 2 concepts were the basis for ideas about heredity
Heredity occurs within species
Traits are transmitted directly from parent to offspring
Thought parents made equal genetic contributions and traits are blended in offspring
Paradox – if blending occurs why don’t all individuals look alike?

4. Early work
Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents
Additional variation observed in 2nd generation offspring contradicts direct transmission
T.A. Knight – 1823 – crossed 2 varieties of garden pea, Pisum sativa
Crossed 2 true-breeding strains
1st generation resembled only 1 parent strain
2nd generation resembled both

5. Particular inheritance
The idea that the determinants of hereditary traits (genes) are transmitted intact from one generation to the next
Proposed by Gregor Mendel.

6. Gregor Mendel Austrian Monk (1822 - 1884)
Studied physics, math and botany.
“Father of genetics”
Laid the foundation for modern genetics.
Hypothesis for blending inheritance is wrong.

7. Hybridization – when 2 individuals with different characteristics are mated or crossed to each other.
The offspring are hybrids.
Mendel carried out systematic, quantitative hybridization experiments in which he analyzed the numbers of offspring carrying specific traits.

8. Gregor Mendel Chose to study pea plants because:
Other research showed that pea hybrids could be produced
Many pea varieties were available
Peas are small plants and easy to grow
Peas can self-fertilize or be cross-fertilized

10. Mendel’s experimental method
Usually 3 stages
Produce true-breeding strains for each trait he was studying
Cross-fertilize true-breeding strains having alternate forms of a trait
Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

12. True breeding - a variety that continues to exhibit the same trait after several generations of self fertilization.
Monohybrid crosses - Cross to study only 2 variations of a single trait.
Mendel produced true-breeding pea strains for 7 different traits. Each trait had 2 variants (ex. Color - purple, white)
P generation - parental generation - true breeding parents.

13. F1 generation First filial generation
Offspring produced by crossing 2 true-breeding strains
For every trait Mendel studied, all F1 plants resembled only 1 parent
Referred to this trait as dominant
Alternative trait was recessive
No plants with characteristics intermediate between the 2 parents were produced

14. F2 generation Second filial generation
Offspring resulting from the self-fertilization of F1 plants
Although hidden in the F1 generation, the recessive trait had reappeared among some F2 individuals
Counted proportions of traits
Always found about 3:1 ratio

16. 3:1 is 1:2:1 F2 plants (phenotype)
¾ plants with the dominant form
¼ plants with the recessive form
The dominant to recessive ratio was 3:1
Mendel discovered the ratio is actually: (genotype)
1 true-breeding dominant plant
2 not-true-breeding dominant plants
1 true-breeding recessive plant

18. Conclusions His plants did not show intermediate traits
Each trait is intact, discrete
For each pair, one trait was dominant, the other recessive
Pairs of alternative traits examined were segregated among the progeny of a particular cross
Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive

19. 5 element model Parents transmit discrete factors (genes)
Each individual receives one copy of a gene from each parent
Not all copies of a gene are identical
Allele – alternative form of a gene
Homozygous – 2 of the same allele
Heterozygous – different alleles

20. Alleles remain discrete – no blending
Presence of allele does not guarantee expression
Dominant allele – expressed
Recessive allele – hidden by dominant allele
Genotype – genetic composition of an individual (TT, tt)
Phenotype – physical appearance (TT - tall, tt - short)

21. Principle of Segregation
Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization
Physical basis for allele segregation is the behavior of chromosomes during meiosis
Mendel had no knowledge of chromosomes or meiosis – had not yet been described

22. Punnett square Cross purple-flowered plant with white-flowered plant
P is dominant allele – purple flowers
p is recessive allele – white flowers
True-breeding white-flowered plant is pp
Homozygous recessive
True-breeding purple-flowered plant is PP
Homozygous dominant
Pp is heterozygote purple-flowered plant

25. Dihybrid crosses Examination of 2 separate traits in a single cross
Produced true-breeding lines for 2 traits
RR YY x rryy
The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait
Allow F1 to self-fertilize to produce F2

26. The F2 generation shows all four possible phenotypes in a set ratio
F1 self-fertilizes
RrYy x RrYy
The F2 generation shows all four possible phenotypes in a set ratio
9:3:3:1
R_Y_:R_yy:rrY_:rryy
Round yellow:round green:wrinkled yellow:wrinkled green

29. Principle of independent assortment
In a dihybrid cross, the alleles of each gene assort independently
The segregation of different allele pairs is independent
Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs

30. Probability Rule of addition
Probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities
When crossing Pp x Pp, the probability of producing Pp offspring is
probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4)
¼ + ¼ = ½

31. Rule of multiplication
Probability of 2 independent events occurring simultaneously is the product of their individual probabilities
When crossing Pp x Pp, the probability of obtaining pp offspring is
Probability of obtaining p from father = ½
Probability of obtaining p from mother = ½
Probability of pp= ½ x ½ = ¼

32. Testcross
Cross used to determine the genotype of an individual with dominant phenotype
Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

34. Extensions to Mendel Mendel’s model of inheritance assumes that
Each trait is controlled by a single gene
Each gene has only 2 alleles
There is a clear dominant-recessive relationship between the alleles
Most genes do not meet these criteria

35. Polygenic inheritance
Occurs when multiple genes are involved in controlling the phenotype of a trait
The phenotype is an accumulation of contributions by multiple genes
These traits show continuous variation and are referred to as quantitative traits
For example – human height
Histogram shows normal distribution

37. Pleiotropy Multiple phenotypic effects of a single allele.
Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions
This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. Multiple symptoms can be traced back to one defective allele.
Cystic fibrosis – multiple symptoms (phenotypes = clogged blood vessels, salty sweat, liver and pancreas failure, etc.) can be tracked back to a single gene defect.

38. Incomplete dominance
Heterozygote is intermediate in phenotype between the 2 homozygotes
Red flowers x white flowers = pink flowers

40. Multiple alleles May be more than 2 alleles for a gene in a population
ABO blood types in humans
3 alleles
Each individual can only have 2 alleles
Number of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an outbreeding population

41. Codominance Heterozygote expresses phenotypes of both dominate alleles
Type AB blood

42. Human ABO blood group The system demonstrates both Multiple alleles
3 alleles of the I gene (IA, IB, and i)
Codominance
IA and IB are dominant to i but codominant to each other

44. Question: Could a child with blood type O be produced from parents with blood types A and B? Question: A mother and her child have blood type O. Which blood group can the father NOT belong to?

45. Pedigrees and Human traits
Some human traits are controlled by a single gene
Some of these exhibit dominant and recessive inheritance
Pedigree analysis is used to track inheritance patterns in families
Dominant pedigree – juvenile glaucoma
Disease causes degeneration of optic nerve leading to blindness
Dominant trait appears in every generation

48. Recessive pedigree – albinism
Condition in which the pigment melanin is not produced
Pedigree for form of albinism due to a nonfunctional allele of the enzyme tyrosinase
Males and females affected equally
Most affected individuals have unaffected parents

50. Environmental influence
Coat color in Himalayan rabbits and Siamese cats
Allele produces an enzyme that allows pigment production only at temperatures below 30oC

51. Epistasis
The action of one gene is modified or controlled by several genes.