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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Dominance Relations Complete dominance Incomplete dominance Codominance

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

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

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

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

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

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