1. Chapter 9 Patterns of Inheritance
Mendelian Genetics
Chapter 9
Patterns of Inheritance

2. Gregor Mendel
1850s, Mendel studied inheritance in pea plants by breeding them
Peas are good because they have distinctive traits, male and female parts, and are easy to manipulate
He crossed true breeding with other true breeding as well as with hybrids

3. Gregor Mendel P = parental generation
F1 = First filial generation (P X P)
F2 = Second filial (F1 X F1)
True breeding (homozygous) = plants that only breed to produce one phenotype
Hybrid (heterozygous) = results of crosses between two plants that breed true for different phenotypes for the same trait

4. Gregor Mendel Genotype = genetic makeup of an organism
Sequence of the gene
Phenotype = expressed genotype
Visible or measurable traits – based upon genotype

5. Gregor Mendel
He crossed a true breeding purple-flowered plant with a true breeding white-flowered plant
All of the F1 generation was purple

6. Gregor Mendel He then crossed two F1’s with each other
The results (F2’s) were roughly 3 purples to 1 whites
White trait was absent in F1’s but reappeared in the F2’s

7. Gregor Mendel
The reappearance of the white trait meant that it was not altered in the F1, just “hidden”
Mendel did the same crosses, and got similar results, with other traits of the pea plant

9. Gregor Mendel

10. Gregor Mendel Four ideas spawn from Mendel’s work:
Alleles account for different traits
Organisms inherit two alleles, one from each parent (may be same or different)
If alleles are different, then one is dominant over the other
The two alleles segregate (law of segregation) during gamete formation

11. Punnett Square
Predicts the statistical results of a cross

12. Punnett Square
A test cross, breeding a homozygous recessive with a dominant phenotype, but unknown genotype, can determine the identity of the unknown allele

13. Punnett Square One Factor Crosses = crosses with only one trait
Monohybrid crosses = crossing one heterozygote with another
Two Factor Crosses = crosses with two traits
Dihybrid crosses = crossing one organism that is heterozygous for both traits with another that is heterozygous for both traits

14. Punnett Square
In monohybrid crosses, he noticed that genotypic and phenotypic ratios could be predicted
Genotypic ratio = 1:2:1
Phenotypic ratio = 3:1

15. Punnett Square Example of a dihybrid cross:
Crossing one plant that is heterozygous for seed shape and yellow seed coat (RrYy) with another plant that is also heterozygous for both traits (RrYy)
RrYy X RrYy
Dihybrid results point to the law of independent assortment

16. Independent Assortment
In our example (RrYy X RrYy), both parents can produce four possible gametes RY Ry rY
Since both parents have four possible gametes, there are 16 possible combinations for fertilization

17. Independent Assortment

18. Independent Assortment
These possible combinations produce a genotypic ratio of 1:2:1:2:4:2:1:2:1
It also produces a phenotypic ratio of 9:3:3:1
The law of independent assortment simply means that during gamete formation, allelic pairs that code for different traits assort independent of each other

19. Dominance Mendel was very lucky to have studied peas
They follow simple dominance laws
Not all cells follow the simple laws of dominance vs. recessiveness

20. Variations
Incomplete dominance = neither allele is shown in its dominant form; an intermediate phenotype is shown
Genotypic and phenotypic ratios are the same (1:2:1)

21. Variations
Codominance = heterozygote expresses a phenotype that is distinct from and not intermediate between those of the two homozygotes
Ex. Human AB blood type

22. Variations
Codominance

23. Variations
Multiple alleles – gene exists as more than two alleles in the population
Rabbit coat color gene has 4 alleles: C, c, cch & ch
5 phenotypes
10 genotypes

24. Variations
Pleiotropy – one gene affects more than one phenotypic characteristic
Ex: sickle-cell disease affects much more than just the overall conformation of the hemoglobin protein

25. Variations
Epistasis – one gene at one locus affects the phenotype of another gene at another locus
Ex: mice coat color depends on two genes
One, the epistatic gene, determines whether or not pigment will be deposited in hair
Presence (C) is dominant to absence (c)
The second determines whether the pigment to be deposited is black (B) or brown (b)
An individual that is cc has a white (albino) coat regardless of the genotype of the second gene

26. Variations – epistasis
A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment
However, unlike the 9:3:3:1 offspring ratio of an normal Mendelian experiment, the ratio is 9 black, brown, and 4 white

27. Variations
Polygenic – one phenotype determined by the combined effect of more than one gene
Ex: human skin color, eye color, height

28. Variations

29. Variations
Multifactorial Traits – determined by the combined effect of one or more genes plus the environment
Ex: heart disease, body weight, intelligence

30. Pedigrees Autosomal Recessive Traits: located on non-sex chromosomes
affects males and females
parents must be carriers or affected
affected individuals are homozygous recessive
Ex: Albinism, Cystic fibrosis, Phenylketonuria, Sickle cell disease

31. Autosomal Recessive

32. Pedigrees Autosomal Dominant Traits: located on non-sex chromosomes
affects males and females
at least one parent is affected
does not skip generations
affected individuals are homozygous dominant or heterozygous
Ex. Achondroplasia, Huntington disease, Lactose intolerance, Polydactyly

33. Autosomal Dominant