Lecture 13: May 24, 2004 CH14: Mendel and the gene idea *particulate inheritance parents pass on discrete heritable units *gene- unit of inheritance which occupies a specific chromosomal location (locus) *allele- alternative forms of a single gene Mendel s hypothesis (to explain his results) 1. Alternative versions of genes account for variation in inherited characters 2. For each character, an organism inherits two alleles, one from each parent 3. If two alleles differ, one is dominant, the other recessive 4. The two alleles for each character segregate (separate) during gamete production. *genetic make-up genotype *appearance - phenotype *Homozygous 2 identical alleles for a character *Heterozygous 2 different alleles for a character
Sample problem Albinism in humans is inherited as a simple recessive trait. Determine the genotypes of the parents and offspring for the following families. When two alternative genotypes are possible, list both. (A) Two non albino (normal) parents have five children, four normal and one albino. (B) A normal male and an albino female have six children, all normal. 1) establish gene symbols: 2) Establish: genotype phenotype Move on to part (A): Parents are both phenotypically normal, genotypes could be EITHER AA or Aa, an albino phenotype could only result from an aa genotype. *One a had to come from the mother and one a had to come from the father, so, the parents must be genotypically Aa.
Sample problem con t: (B) A normal male and an albino female have six children, all normal. 1) The female is phenotypically albino; genotype can only be aa 2) The male is phenotypically normal; genotype can be AA or Aa 3) Since all children are normal one might assume the male to be AA 4) BUT male COULD also be Aa! *IF the father was genotypically Aa, then what is the likelihood (chance or probability) of this couple having 6 normal children? Recall the product law!
Mendel s Law of Independent Assortment *What happened when he looked at two characters? If they segregate together: If they segregate independently:
Dihybrid cross- A genetic cross between two individuals involving two characters Example: Punnett square and the law of independent assortment: P 1 X yellow, round GGWW green, wrinkled ggww F 1 All yellow, round GgWw
Punnett square and the law of independent assortment: F 1 F 1 X All yellow, round GgWw All yellow, round GgWw F 2 9/16 yellow, round 3/16 yellow, wrinkled 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 3/16 green, round 1/16 ggww, 2/16 ggww 1/16 green, wrinkled 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
The Rule of Addition *The rule of addition *The dominant allele could come from the sperm and the recessive from the ovum Probability = 1/4 *The dominant allele could come from the ovum and the recessive from the sperm Probability = 1/4 *The probability of a heterozygote = ¼ + ¼ = ½
Modifications of Mendelian Ratios *Incomplete dominance *Codominance Example: MN Blood group *We can predict genotypic and phenotypic ratios
Modifications of Mendelian Ratios *Multiple Alleles Examples: *100 alleles at a given locus in Drosophila *ABO Blood group in humans *Pleiotropy Genotype I A I A I A I O I B I B I B I O I A I B I O I O Antigen A A B B A,B Neither Phenotype A A B B AB O
Modifications of Mendelian Ratios *Epistasis Example: In Drosophila, gene:eyeless *Black (B) is dominant to brown (b) *Second gene responsible for allowing pigment to be deposited in hair C = presence, c = absence (colorless)
*Quantitative characters *often due to polygenic inheritance Example- *skin color controlled by 3 genes *each gene with two alleles light and dark incomplete dominance
Phenotype depends on environment and genes CH15: The Chromosomal basis of inheritance *We will cover pgs 269-270 (Mendel and chromosomes) and pgs 276-279 (sex chromosomes)
Sex Chromosomes *1909 Thomas Hunt Morgan II III IV XY or XX Sex chromosomes Autosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male
*Each chromosome has two regions: *homologous region *differential region Sex-linkage X-linkage Transmission of sex-linked recessive traits: Sex-linked disorders in humans *Duchenne muscular dystrophy *Hemophilia
The X Chromosome and Dosage Compensation *Do females produce twice as much of each gene product for X-linked genes? Dosage compensation *In mammals X inactivation