1. Genetics: an Introduction

2. Genetics: the study of Heredity
Heredity: transmission of traits from one generation to the next
Gregor Mendel: father of modern genetics
worked in 1860’s
bred garden peas
used applied mathematics:
statistics!!!!

3. Genetics character: observable physical feature
flower color, eye color, etc
trait: form of character
purple or white flowers; brown or blue eyes
heritable trait: passed from parent to offspring

4. Garden Peas: who knew?
Stamen
Carpel
Petal

6. Parents (P) Offspring (F1)
Figure 9.2C_s3
White 1
Removal of stamens
Stamens
Carpel 2
Transfer of pollen
Parents (P)
Purple 3
Carpel matures into pea pod 4
Seeds from pod planted
Figure 9.2C_s3 Mendel’s technique for cross-fertilization of pea plants (step 3)
Offspring (F1) 6

7. Genetics
True-breeding varieties result when self-fertilization produces offspring all identical to the parent.
The offspring of two different varieties are hybrids.
The cross-fertilization is a hybridization,
or genetic cross.
True-breeding parental plants are the P generation.
Hybrid offspring are the F1 generation.
A cross of F1 plants produces an F2 generation.

8. Figure 12.6 Inheritance of a Single Trait over Three Generations
Mendel crossed parent plants that were true-breeding (homozygous) for two discrete phenotypes (purple or white) of a particular trait (flower color). Such breeding trials are described as monohybrid crosses because the F1 plants are hybrid (heterozygous) for a single trait (flower color).

9. Genetics: Mendel (monohybrid cross)
The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis.
Mendel needed to explain why
white color seemed to disappear in the F1 generation and
white color reappeared in one quarter of the F2 offspring.
Mendel observed the same patterns of inheritance for six other pea plant characters.

10. Genetics: Mendel (monohybrid cross)
Mendel developed four hypotheses, described below using modern terminology.
1. Alleles are alternative versions of genes that account for variations in inherited characters.
2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.
A homozygous genotype has identical alleles.
A heterozygous genotype has two different alleles.

11. Genetics: Mendel (monohybrid cross)
If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.
The phenotype is the appearance or expression of a trait.
The genotype is the genetic makeup of a trait.
The same phenotype may be determined by more than one genotype.

12. Genetics: Mendel (monohybrid cross)
A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.
Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation.
The F1 hybrids all have a Pp genotype.
A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine

13. Figure 12.7 The Punnett Square Method Is Used to Predict All Possible Outcomes of a Genetic Cross
Punnett squares chart the segregation (separation) of alleles into gametes and all the possible ways in which the alleles borne by these gametes can be combined to produce offspring.

14. Phenotypic ratio 3 purple : 1 white P PP Pp Eggs from F1 plant
Figure 9.3B_4
F2 generation
Sperm from F1 plant P p
Phenotypic ratio 3 purple : 1 white P PP Pp
Eggs from F1 plant
Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp
Figure 9.3B_4 An explanation of the crosses in Figure 9.3A (F2 generation) 14

15. Genetics: Test Cross
if you have an individual with the dominant phenotype: you CANNOT know its genotype
( Black = B, b = brown)
Black phenotype could be BB or Bb genotype
How do you find out (without DNA testing)?
do a TEST CROSS
an y individual with the recessive phenotype MUST have the homozygous recessive genotype

16. What is the genotype of the black dog?
Figure 9.6
What is the genotype of the black dog?
Testcross 
Genotypes
B_? bb
Two possibilities for the black dog: BB or Bb
Figure 9.6 Using a testcross to determine genotype
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate 16

17. B B b Bb Bb b Bb Bb Genetics: Test Cross if the Black dog is BB
phenotype of ALL offspring = Black
genotype of ALL offspring = heterozygous b Bb Bb b Bb Bb

18. B b b Bb bb Bb bb b Genetics: Test Cross if Black dog is Bb
phenotype of HALF offspring = Black
phenotype of HALF offspring = brown b Bb bb Bb bb b

19. Genetics: Dihybrid Cross
looking at two traits at the same time
start with ‘true breeding’ plants for BOTH traits
R = round; r = wrinkled
Y = yellow; y = green
RRYY (homozygous Dominant for both traits) crossed with
rryy (homozygous recessive for both traits) = R generation
F1 generation ALL = RrYy (round and yellow)
cross F1 with F1 to get F2 generation

20. Figure 12.8 Inheritance of Two Traits over Three Generations
Mendel used two-trait breeding experiments to test the hypothesis that the alleles of two different genes are inherited independently from each other. In one set of experiments, illustrated here, Mendel tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. The real test of the hypothesis came when Mendel examined the phenotypes of the offspring produced by crossing the heterozygous F1 plants (RrYy). As predicted by the hypothesis, two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes. A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is called a dihybrid cross.

21. Figure 12.8 (Part 1) Inheritance of Two Traits over Three Generations
Mendel used two-trait breeding experiments to test the hypothesis that the alleles of two different genes are inherited independently from each other. In one set of experiments, illustrated here, Mendel tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. The real test of the hypothesis came when Mendel examined the phenotypes of the offspring produced by crossing the heterozygous F1 plants (RrYy). As predicted by the hypothesis, two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes. A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is called a dihybrid cross.

22. Figure 12.8 (Part 2) Inheritance of Two Traits over Three Generations
Mendel used two-trait breeding experiments to test the hypothesis that the alleles of two different genes are inherited independently from each other. In one set of experiments, illustrated here, Mendel tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. The real test of the hypothesis came when Mendel examined the phenotypes of the offspring produced by crossing the heterozygous F1 plants (RrYy). As predicted by the hypothesis, two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes. A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is called a dihybrid cross.

23. Figure 12.8 (Part 3) Inheritance of Two Traits over Three Generations
Mendel used two-trait breeding experiments to test the hypothesis that the alleles of two different genes are inherited independently from each other. In one set of experiments, illustrated here, Mendel tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. The real test of the hypothesis came when Mendel examined the phenotypes of the offspring produced by crossing the heterozygous F1 plants (RrYy). As predicted by the hypothesis, two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes. A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is called a dihybrid cross.

24. Figure 12.8 (Part 4) Inheritance of Two Traits over Three Generations
Mendel used two-trait breeding experiments to test the hypothesis that the alleles of two different genes are inherited independently from each other. In one set of experiments, illustrated here, Mendel tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. The real test of the hypothesis came when Mendel examined the phenotypes of the offspring produced by crossing the heterozygous F1 plants (RrYy). As predicted by the hypothesis, two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes. A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is called a dihybrid cross.

25. Genetics: Dihybrid Cross
F2 generation: NEW COMBINATIONS!!
9/16 had round yellow seeds
3/16 had wrinkled yellow seeds
3/16 had round green seeds
1/16 had wrinkled green seeds
Independent assortment occurred
(genes assort independently into gametes)

26. F2 Generation Genotypes
RRYY = 1/16 round yellow RRYy = 2/16 round yellow Rryy = 1/16 round green RrYY = 2/16 round yellow RrYy = 4/16 round yellow Rryy = 2/16 round green rrYY = 1/16 wrinkled yellow rrYy = 2/16 wrinkled yellow rrYy = 1/16 wrinkled green