Patterns of Inheritance

1. Patterns of Inheritance Chapter 9

3. Just 1 of 9 different types of muscular dystrophy – a group of genetic, degenerative disorders mainly affecting skeletal muscles • X-linked recessive mutation in the dystrophin gene - absence of dystrophin, a structural protein for muscle fibers

4. Biology and Society: Testing Before Birth • Genetic testing before birth allows expectant parents to know their offspring’s genetic makeup - increased risk of passing on a genetic disease • Amniocentesis – performed between weeks 14-20 - a needle is inserted through the mother’s abdomen into her uterus to obtain ~10 mls of amniotic fluid - extract fetal DNA from the skin cells shed by the fetus • Chorinic villus sampling (CVS) – 8th week - narrow, flexible tube is inserted through the mother’s vagina and into her uterus - small amount of chorionic (placental) tissue is removed and tested

5. Aminocentesis: cells collected from the fluid can be genetically tested Figure 9.1

6. Heritable Variation and Patterns of Inheritance • Gregor Mendel – a monk in Austria (Czech Republic) was the 1st person to analyze patterns of inheritance - deduced the fundamental laws of genetics by conducting experiments with garden peas - bred plants with different characteristics (flower color or seed shape) - in 1866, published a paper where he argued that parents pass on to their offspring discrete ‘heritable factors’

7. Figure 9.2

8. In an Abbey Garden • Mendel studied garden peas - easy to grow with easily distinguishable varieties • Petals enclose the egg-and sperm-producing parts - the carpel and stamens - capable of self-fertilization pollen grains released from the stamens land on the tip of the carpel on the same flower - ensured self-fertilization by covering a flower with a small bag, pollen from another plant could not reach the carpel - for cross-fertilization (fertilization of one plant by pollen from a different plant) Mendel pollinated the plants by hand - with a known source of pollen, Mendel could always be sure of the parentage of his new plants - selected characteristics that occurred in 2 distinct forms

9. Structure of a Pea Flower • The carpel produces egg cells • The stamens make pollen, which carries sperm Figure 9.3

10. True-breeding (purebred) - varieties for which self – fertilization produced offspring identical to the parent - identified a purple-flowered variety when self-fertilized always produced offspring plants with purple flowers • Question: what would happen if Mendel crossed different true-breeding varieties with each other - example: what offspring would result if plants with purple flowers were cross-fertilized with plants with white flowers - offspring of 2 different true-breeding varieties are hybrids - cross-fertilization is a genetic cross - parental plants, the P generation; their hybrid offspring the F1 generation (filial ‘son’ or ‘daughter’) - F1 plants self-fertilize or cross-fertilize each other, their offspring are the F2 generation

11. 1. To prevent self-fertilization, Mendel cut off the stamens from an immature flower before they produced pollen - stamenless plant would be the female parent 2. To cross-fertilize this female, he dusted its carpel with pollen from another plant. After pollination, 3. the Carpel developed into a pod, contianing seeds (peas) 4. He planted the seeds, and they grew into offspring plants Fig 9.4

12. Mendel’s Law of Segregation • Mendel performed many experiments in which he tracked the inheritance of characteristics - characteristics, that occur as 2 alternative traits • Mendel created true-breeding varieties of plants - then crossed 2 different true-breeding varieties • His results led him to formulate several hypotheses about inheritance

13. 7 characteristics studied by Mendel • Each comes in 2 alternative forms • One alternative is dominant • One is recessive Figure 9.5

14. Monohybrid Crosses • A cross between parent plants that differ in only one characteristic - Mendel started with a cross between a pea plant with purple flowers and one with white flowers - all the F1 plants had purple flowers - was the heritable factor lost for the white flower? - Mendel mated the F1 plants with each other - he found in the F2 plants, ~1/4 had white flowers

15. The law of segregation • Crossing true-breeding purple-flowered plants with true-breeding white-flowered plants • Produced F1 plants with purple flowers • Self-fertilization of F1 plants • Produced 929 F2: 709 with purple flowers 224 with white flowers Figure 9.6a

16. Mendel developed 4 hypotheses from the monohybrid cross: 1. There are alternative forms of genes (inheritable factors), called alleles 2. For each inherited characteristic, an organism inherits 2 alleles, one from each parent. - alleles may be the same or different 3. If the 2 alleles of an inherited pair differ, one the dominant allele determines the appearance (recessive allele) 4. Gametes carry only one allele for each inherited characteristic because the 2 members of an allele pair segregate (separate) from each other during the production of gametes This hypothesis is now known as the Law of Segregation

17. Explanation of the results in part a (uppercase letter – dominant allele) • Parental plants – true breeding • Gametes (circled) have only one allele • Union of the parental gametes produced F1 hybrids (Pp) • 2 alleles segregated, ½ the gametes received the P allele, other ½ a p allele • Punnet square: F2 plants – 3 : 1 Fig 9.6b

18. What will be the physical appearance of the F2 offspring? - an offspring with an uppercase P (dominant trait) will have purple flowers - only an offspring with 2 lowercase pp (recessive trait) will have white flowers • An organism’s appearance does not always reveal its genetic composition, - geneticists distinguish between expressed, or physical traits, or phenotype and - an organism’s genetic makeup or genotype

19. Genetic Alleles & Homologous Chromosomes • A pair of homologous chromosomes (homologues) - chromosomes that carry alleles of the same genes - one member comes from the organism’s female parent - other member comes from the male parent - alleles, alternate forms of a gene reside at the same locus on each homologous chromosome

20. Labeled bands represent 3 gene loci (sing., locus) – specific locations of genes along the chromosome • Alleles ( alternative forms) of a gene reside at the same locus on homologous chromosomes • 2 Chs bear identical alleles for a gene – homozygous • 2 Chs bear 2 different alleles for a gene - heterozygous Figure 9.7