1. Genes Units of information about specific traits
Passed from parents to offspring
Each has a specific location (locus) on a chromosome

2. Alleles
Different molecular forms of a gene found on homologous chromosomes
Arise by mutation
Dominant allele masks a recessive allele that is paired with it

3. Allele Combinations Homozygous Heterozygous
having two identical alleles
Homozygous dominant, AA
Homozygous recessive, aa
Heterozygous
having two different alleles Aa

4. homozygous dominant parent homozygous recessive parent
(chromosomes duplicated before meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization produces heterozygous offspring
Fig. 8-4, p.114

5. Average F2 dominant-to-recessive ratio for all of the traits studied:
Dominant Form
Recessive Form
FLOWER COLOR
705 purple
224 white
3.15:1
FLOWER POSITION
651 along stem
207 at tip
3.14:1
STEM LENGTH
787 tall
227 dwarf
2.84:1
Average F2 dominant-to-recessive ratio for all of the traits studied:
3:1
Fig. 8-5, p.115

6. Probability and the Punnett Square
female gametes a A aa A a A a A a A A A Aa AA Aa
male gametes a Aa aa a Aa aa a Aa aa
Fig. 8-6a, p.115

7. Experimental intercross between
Monohybrid Cross
Experimental intercross between
two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa ?

8. A Monohybrid Cross aa Aa AA A a True-breeding homozygous recessive
parent plant
homozygous dominant
An F1 plant
self-fertilizes
and produces
gametes:
F1 PHENOTYPES
F2 PHENOTYPES aa Aa AA A a
A Monohybrid Cross

9. Mendel’s Theory of Segregation
Individual inherits a unit of information (allele) for a trait from each parent
During gamete formation, the alleles segregate from each other

10. Dihybrid Cross
AB X ab
Experimental cross between individuals that are heterozygous for different versions of two traits

11. Dihybrid Cross: F1 Results
purple
flowers, tall
white
flowers,
dwarf
TRUE-
BREEDING
PARENTS:
AABB x
aabb
GAMETES: AB AB ab ab
AaBb
F1 HYBRID
OFFSPRING:
all purple-flowered, tall

12. 3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall
AABB
purple-
flowered,
tall parent
(homozygous
dominant) 2
aabb
white-
flowered,
dwarf parent
(homozygous
recessive) AB X ab
3 F1 OUTCOME: All of the F1 plants are purple-flowered, tall
(AaBb heterozygotes)
Fig. 8-7, p.116

13. meiosis, gamete formation meiosis, gamete formation
AaBb
AaBb
meiosis,
gamete formation
meiosis,
gamete formation
Fig. 8-7, p.116

14. Dihybrid Cross: F2 Results
AaBb X
AaBb
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
9/16 purple-flowered, tall
1/4 AB
1/16
AABB
1/16
AABb
1/16
AaBB
1/16
AaBb
3/16 purple-flowered, dwarf
3/16 white-flowered, tall
1/4 Ab
1/16
AABb
1/16
AAbb
1/16
AaBb
1/16
Aabb
1/16 white-flowered, dwarf
1/4 aB
1/16
AaBB
1/16
AaBb
1/16
aaBB
1/16
aaBb
1/16
AaBb
1/16
Aabb
1/16
aaBb
1/16
aabb
1/4 ab

15. Independent Assortment
“Units” for one trait were assorted into gametes independently of the “units” for the other trait
Members of each pair of homologous chromosomes are randomly sorted into gametes during meiosis

16. Independent Assortment
Metaphase I: OR A A a a A A a a B B b b b b B B
Metaphase II: A A a a A A a a B B b b b b B B
Gametes: B B b b b b B B A A a a A A a a
1/4 AB
1/4 ab
1/4 Ab
1/4 aB

17. Alleles Different molecular forms of a gene Arise through mutation
Diploid cell has a pair of alleles at each locus
Alleles on homologous chromosomes may be same or different

18. The Y Chromosome Small, with few genes
Master gene for male sex determination
SRY gene (sex-determining region of Y)
SRY present, testes form
SRY absent, ovaries form

19. The X Chromosome Carries more than 2,000 genes
Most genes deal with nonsexual traits
Genes on X chromosome can be expressed in both males and females

20. Dominance Relations Complete dominance Incomplete dominance
Codominance

21. Recessive traits Recessive inheritance
Two recessive alleles are needed to show disease
Heterozygous parents are carriers of the disease-causing allele
Probability of inheritance increases with inbreeding, mating between close relatives

22. Dominant Traits Dominant inheritance
One dominant allele is needed to show disease
Dominant lethal alleles are usually eliminated from the population

23. Codominance Codominance Neither allele is dominant over the other
Expression of both alleles is observed as a distinct phenotype in the heterozygous individual
Observed for type AB blood

24. Incomplete dominance Incomplete dominance
Neither allele is dominant over the other
Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual

25. Autosomal Recessive Inheritance Patterns
If parents are both heterozygous, child will have a 25% chance of being affected

26. X-Linked Recessive Inheritance
Males show disorder more than females
Son cannot inherit disorder from his father