Mendel and the Gene Idea: Laws of Inheritance and Probability

1. Chapter 14 Mendel and the Gene Idea

2. Introduction to Mendel • Austrian monk who crossed pea plants to determine that there are two alleles for each trait; dominant and recessive • Each sex cell only carries one allele for each trait (independent assortment of chromosomes) • Mendel used true breeding homozygous parents for the P (parental) generation • Monohybrid cross produces a 3:1 phenotypic ratio of |dominant:recessive|; then produces a 1:2:1 genotypic ratio of |homozygous dominant:heterozygous:homozygous recessive|

3. Law of Segregation • Allele pairs segregate during meiosis II and the paired condition is restored by fertilization. • law predicts a 3:1 ratio in monohybrid cross; can be found using punnet squares • Testcross - cross used to determine genotype of an organism using a homozygous recessive organism. • Homozygous recessive is used for the following reasons: • A homozygous dominant organism will only produce dominant trait phenotypes • A heterozygous dominant organism will produce both dominant and recessive phenotypes.

4. Law of Independent Assortment • Each allele pair segregates independent of other gene pairs during meiosis • Dihybridcross- produced four different phenotypes with many different genotypes • Able to conclude that allele segregation is not associated with other allele segregation; seed color has no affiliation to seed size

5. Law of Probability • LawofProbability – scale used to predict random outcomes if the genotypes are known • scale ranges from 0-1; event that is certain to occur has a probability of 1 • probabilities of all events must add up to 1 • outcome of a random event is unaffected by the outcome of previous events

6. Law of Probability (continued) • Rule of Multiplication – the probability that independent events will occur simultaneously is the product of their individual probabilities. • Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive? • Answer: Probability that egg from the F1 (Pp) will have (p) allele = ½ • Probability that sperm from the F1 (Pp) will have (p) allele = ½ • Overall probability: ½ x ½ = ¼

7. Rule of Probability (continued) • Rule of Addition – the probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. • Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being a heterozygote? • Answer:There are two ways that a heterozygote may be produced. The dominant allele may be given by either the sperm or the egg and the recessive allele may be given by the sperm or the egg • Probability that the dominant allele will be in the sperm and the recessive allele will be in the egg = ½ x ½ = ¼ • Probability that the recessive allele will be in the sperm and the dominant allele will be in the egg = ½ x ½ = ¼ • Probability that a heterozygote will be produced: ¼ + ¼ = ½

8. Using Probability to Solve Problems • Question: What is the probability that a trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc will produce an offspring with the genotype aabbcc? • The segregation of alleles is an independent event, this can be treated as three separate monohybrid crosses. • Aa x Aa probability for aa offspring = 1/4 • Bb x Bb probability for bb offspring = 1/4 • Cc x Cc probability for cc offspring = 1/4 • The probability that all of these events must occur simultaneously is the product of their independent probabilities (rule of multiplication). • 1/4aa x 1/4bb x 1/4cc = 1/64

9. Extending Mendelian Genetics • IncompleteDominance – one allele is not completely dominant over another allele; phenotype appears to be in between; expressed in heterozygote genotype • Completedominance- dominant allele completely masks expression of the recessive allele in the heterozygote. • Codominance- full expression of both alleles in the heterozygote • Tay-Sachsdisease- recessively inherited trait; however, intermediate phenotype of heterozygotes, but do not show symptoms. • Lack lipid metabolizing enzymes, therefore lipids accumulate in the brain. • Multiplealleles- more than just two alternative forms of the gene. (ABO Blood Group) • Alleles for blood type are IA, IB, and i for blood types A,B,AB, and O.

10. Extending Mendelian Genetics (Continued) • Pleiotropy – single gene having multiple phenotypic effects. • gene that controls pigmentation of fur in tigers and Siamese cats also influences connections between the eyes and brain. • PolygenicInheritance – variation of traits is determine by many segregating loci; inheritance is effected by two or more genes. • Skin • GenotypePhenotype • AABBCC Dark • aabbcc Light • AaBbCc Intermediate shade

11. Pedigree Analysis • Pedigree – a family tree that diagrams relationships among parents and children across generations to show the inheritance pattern of a particular phenotype. • The pedigree below is tracing a dominant allele for the expression of a widow’s peak.

12. Patterns of Inheritance • Recessively Inherited Disorders- usually code for a malfunctioning protein or no protein at all. • Cysticfibrosis – accumulation of thickened mucus in pancreas, intestinal tract and lungs, and bacterial infections due to defective chloride ion gated channels in the brain. • Tay-Sachs – can not metabolize lipids • Sickle-CellAnemia – caused by single amino acid substitution in the protein hemoglobin; does not allow blood oxygen levels to reach the highest capacity. • Dominantly Inherited disorders • Huntington’sDisease – degenerative disease of the the nervous system. • Multifactorial Disorders- influence by inheritance and the environment. (Heart Disease, Diabetes, Cancer, Alcoholism)

13. Technology • Fetal Testing • Ultrasound – identify structural problems • ChorionicvillusSampling – suctioning off of a small amount of fetal tissue for karyotyping. • Amniocentesis – amniotic fluid testing