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Genetics Purple flowers F 1 Generation White flowers P Generation (true-breading parents) All plants have purple flowers F 2 Generation Fertilization among F 1 plants (F 1  F 1 ) of plants have purple flowers of plants have white flowers

Genetics Mendel developed four hypotheses from the monohybrid cross: 1. There are alternative versions of genes, called alleles. 2. For each character, an organism inherits two alleles, one from each parent. –An organism is homozygous for that gene if both alleles are identical. –An organism is heterozygous for that gene if the alleles are different.

Genetics 3. If two alleles of an inherited pair differ: The allele that determines the organism’s appearance is the dominant allele The other allele, which has no noticeable effect on the appearance, is the recessive allele 4. Gametes carry only one allele for each inherited character. –The two members of an allele pair segregate (separate) from each other during the production of gametes. –This statement is the law of segregation.

Genetics A Punnett Square predicts the results of a genetic cross between individuals of known genotype Ddd TallDwarf P: DD dd X Gamete formation: DDDddDdDdDdDd All (4/4) are Dd All (4/4) are Tall

Geneticists distinguish between an organism’s physical traits and its genetic makeup. An organism’s physical traits are its phenotype. An organism’s genetic makeup is its genotype. Genetics

Recall: Independent Assortment: Genetics

Dihybrid cross- A genetic cross between two individuals involving two characters GGWWggww Example: P yellow, roundgreen, wrinkled X GW GW gw GgWw Genetics F1F1 All yellow, round

F1F1 GgWw F1F1 All yellow, round GgWw X F2F2 9/16 yellow, round 3/16 yellow, wrinkled 3/16 green, round 1/16 green, wrinkled GW Gw gW gw gW GW gw Gw 9:3:3:1 Phenotypic ratio; Genotypic ratio as follows: 1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw 1/16 GGww, 2/16 Ggww 1/16 ggWw, 2/16 ggWw 1/16 ggww

Mendelian inheritance is based on probability Example- coin toss *1/2 chance landing heads *Each toss is an independent event *Coin toss, just like the distribution of alleles into gametes *The rule of multiplication – determines the chance that two or more independent events will occur together ½ x ½ = ¼ B B b B b b Female gametes B B B b b b Male gametes Formation of sperm Bb male Formation of eggs Bb female F 2 Genotypes F 1 Genotypes (  ) Genetics

Female Male Attached Free Third generation (brother and sister) Second generation (parents, aunts, and uncles) First generation (grandparents) Ff FFff or Ff ff FF or Ff ff Ff ff Genetics: Pedigrees

Human Disorders

F 1 Generation RRrr Gametes P Generation F 2 Generation Sperm Gametes Red White R r Rr Pink R r R r R r RRRr rr Rr Eggs Variations in Mendel’s Laws In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

Homozygous for ability to make LDL receptors Severe disease Mild disease Cell Normal LDL receptor LDL Homozygous for inability to make LDL receptors Heterozygous HH Hh hh GENOTYPE PHENOTYPE Hypercholesterolemia Dangerously high levels of cholesterol in the blood. Is a human trait that is incompletely dominant. Heterozygotes have blood cholesterol levels about 2X normal. Homozygotes have blood cholesterol levels about 5X normal. Variations in Mendel’s Laws

Blood Group (Phenotype) Genotypes Red Blood Cells O A B AB ii IAIBIAIB I B or I B i I A or I A i Carbohydrate A Carbohydrate B Variations in Mendel’s Laws Multiple Alleles and Codominance

Variations in Mendel’s Laws Pleiotropy is the impact of a single gene on more than one character. Pleiotropy Multiple traits (e.g., sickle-cell disease) Single gene

Variations in Mendel’s Laws

Multiple genes Polygenic inheritance Single trait (e.g., skin color) Polygenic inheritance is the additive effects of two or more genes on a single phenotype. Variations in Mendel’s Laws

F 1 Generation P Generation F 2 Generation Sperm AABBCC (very dark) Eggs aabbcc (very light) AaBbCc

Sex Linkage Autosome  Chromosome that is not directly involved in determining the sex of an organism  A chromosome that determines whether an individual is male or female Sex chromosome Female Somatic cells Male 44  XY 44  XX

Any gene located on a sex chromosome is called a sex-linked gene. Most sex-linked genes are found on the X chromosome. Sex Linkage

Genetics and the Environment *Phenotype depends on environment and genes