Patterns of inheritance

1. Patterns of inheritance Genetics &

2. Family resemblance Maternal homologue Paternal homologue Haploid sperm Family resemblance has long been noted and commented on

3. Inheritance – a historical perspective • - Until the middle ages people thought bizarre - composite animals could result from breeding widely different species • - The giraffe (Giraffacamelopardalis) was once thought to have resulted from a camel breeding with a leopard ….say whaaaaat???? Clearly this cannot happen…but how to prove it?

4. Inheritance – a historical perspective • - 1760 - Josef Koelreuter conducted hybridizationexperiments with different strains of tobacco plants and obtained fertile offspring • - Well known animal hybrids include, ligers (lion x tiger), mule (donkey x horse) • - Hybridization may produce fertile or sterile offspring depending on genetic compatibility

5. Blending • During the 19th century the idea of blendingwas popular; traits from parents were thought to be blended together …. But this is false! white horse black horse gray horse

6. How does inheritance actually work? Gregor Mendel’s experiments - Mendel conducted the first quantitative studies of inheritance - Pure breeding adult peas of different flower colors were crossed - Flower color is a trait- an observable characteristic Mendel was an Austrian monk

7. How does inheritance actually work? Gregor Mendel’s experiments Crossing experiment: PurpleX WhitePurple All offspring produced had purple flowers….hmmmm ….. - this disproves the blending theory which would give all lilac coloured flowers! Why were peas used?

8. Mendel used a monohybrid cross What is a monohybrid cross? • Monohybridcross - A cross between two individuals involving to observe inheritance of one trait Hybridization terminology • P generation – True breeding parents • F1 generation – First generation offspring; resulting from a cross between pure breeding individuals (parents) • F2generation – Second generation offspring; resulting from a cross between F1 plants

9. Results of Mendel’s monohybrid cross • F1 generation peas had purple flowers • F2 generation peas consisted of mostly purple flowers (3/4) and small number of white flowers (1/4) • Why? F1 F2

10. So, what is the mechanism of inheritance? • Genes code for traits, each gene has two different forms called alleles • Law of segregation – Alleles separate during meiosis; sperm and egg possess one allele for every gene • Genotype– Combination of alleles one has Mendel’s experiments showed that purple flower color is dominant over white

12. So, what is the mechanism of inheritance? • Genotype– Combination of alleles one has • Two forms of alleles – DOMINANT and recessivealleles • Two alleles = two possible phenotypes • Phenotype– Outward appearance expressed by a gene • Dominant vs. recessive… what’s the difference? Mendel’s experiments showed that purple flower color is dominant over white

13. Punnett square • Letters represent alleles, typically the first letter of a word that defines a trait • Capital P for purple (dominant trait) lowercase p for white (recessive trait) • Genotypes are either homozygous orheterozygous Punnett square

14. Punnett square • Punnett square – A tool developed by Reginald Punnett used to predict the number and variety of genetic combinations • Homozygous– Having the same two alleles for one gene (either both dominant or both recessive) • Heterozygous– Having two different alleles for one gene (one dominant allele, and one recessive allele) Reginald Punnett

15. Dihybrid crossing • Mendel did not know if 2 or more traits were inherited together or separately • If inherited together phenotypic ratio of the F2 generation would be 3:1 (dependent assortment) • In other words, were dominant alleles inherited together and were recessive alleles inherited together?

16. Dihybrid crossing P F1 • Dihybrid cross – Breeding individuals having for to observed inheritance of two different traits • For example, seed color (yellow, green) and seed texture (round, wrinkled) • Independentassortment– each pair of alleles segregates independently of the other pairs of alleles during meiosis F2

17. Both segregation and independent assortment are explained by the distribution of chromosomes during meiosis!!!

18. Other inheritance patterns Mendel’s experiments illustrate completedominance= offspring always resembled one of the two parents Dominant allele had the same phenotypic effect whether present in one or two copies Incompletedominance- heterozygous individuals have an intermediate phenotype Crossing red and white snap dragons produces pink snap dragons

19. Blending inheritance vs. Particulate inheritance F1 F2 Offspring remain pink Return of parental phenotypes Theory is WRONGtheory is RIGHT 

20. Examples of inherited traits in humans • Many human traits are controlled by alleles which seem to be inherited according to Mendellian inheritance laws • Some of the most obvious traits include straight hairline vs. widow’s peak, straight thumb or hitchhiker’s thumb • Are you dominant or recessive for such traits? Refer back to the activity we did in class  Widow’s peak Hitchhiker’s thumb

21. Co-Dominance ex. roan cattle "dominant" traits appear together in the phenotype of hybrid organisms There are red and white hairs present…no blending! Red bull White cow

22. The TEST CROSS! What happens when you know a dominant trait is expressed but you don’t know the genotype…could be BB, or Bb??? WHAT TO DO!!?? Cross it with something of which you know the genotype for SURE. This would have to be a heterozygous individual for the recessive trait…….”bb”

23. Key Points about Test Cross: • 1. the organism with the dominant trait is always crossed with an organism with the recessive trait • 2. if ANY offspring show the recessive trait, the unknown genotype is heterozygous • 3. if ALL the offspring have the dominant trait, the unknown genotype is homozygous dominant • 4. large numbers of offspring are needed for reliable results

24. Some genes have more than one allele…MULTIPLE ALLELES • Blood phenotypes are controlled by a combination of two of three different alleles • Three blood type alleles: i, IA, IB • The alleles for blood types A and B are codominant Blood clumping results when certain antibodies bind to specific antigens Note: i can be written as IO

25. Blood groups and blood types • Erythrocytes (red blood cell) have cell-surface proteins called antigens • Blood types are based upon the types of antigens one has • Defensive chemicals called antibodies are produced by your body to protect itself from foreign cells or organisms • Antibodies circulate through the body in the fluidic blood plasma Type A Type B Type AB Type O

26. Sex linkage Mother A A • Some traits are controlled by alleles found on sex chromosomes • These traits are commonly referred to as either X-linked orY-linked • Use a punnett square to figure out genotypic and phenotypic proportions • Each allele of each sex chromosome written as a superscript a a A A a Father A A Punnett square for sex determination A = dominant allele a = recessive allele

27. Sex-linked disorders and pedigrees Pedigree shows inheritance of traits among family members: Circle = female, Square = male • Sex-linked disorders are caused by genes located on sex chromosomes (usually X chromosome) • Sex-linked disorders and other traits may be traced through a pedigree • Sex-linked disorders include red-green color blindness, hemophilia, and Duchenne muscular distrophy • Are males or females mostly affected by such disorders? Pedigree for normal hearing and deafness

28. Pedigree of a Dominant trait • In these family trees, square symbols represent males and circles represent females. • Shaded symbols represent individuals who exhibit the trait. • A dominant trait such as widow's peak cannot skip a generation. • A recessive trait may skip a generation.

29. Autosomal disorders Dominant disorders - Cause: dominant alleles - Dwarfism (Achondroplasia), Alzheimer’s disease, Huntington’s disease - Less common than recessive disorders Recessive disorders - Cause: recessive alleles - Albinism, Cystic fibrosis, Sickle-cell anemia, Tay-Sachs disease - More common than dominant disorders

30. Human chromosomes • Let’s look at this again • There are 46 chromosomes (23 homologous pairs) in each somatic cell • 22 pairs ofautosomes • 1 pair ofsexchromosomes • XX = Female, XY = Male • Karyotype- chromosomes are arranged according to shape and size Normal human karyotype

31. Nondisjunction and chromosomal disorders • Nondisjunction– failure of chromosomes to separate and segregate into daughter cells • Nondisjunction may occur during meiosis I or meiosis II • Abnormal number of chromosomes may result (such as a trisomydisorder like Down’s syndrome) Nondisjunction can cause more significant problems causing the fetus to die

32. Autosomal nondisjunction-related disorders • - Down’s syndrome (Trisomy-21) • - Patausyndrome (Trisomy-13) • - Edward syndrome (Trisomy-18) Again, individuals with these disorders have an extra chromosome

33. Nondisjunction and chromosomal disorders • Nondisjunction may also affect sex chromosomes • Sex chromosome disorders include Klinefeltersyndrome (XXY, or variants like XXYY, XXXY) in males, and Turner syndrome (XO) in females • These disorders cause poor genital development, and various physical abnormalities such as breast development in men (Klinefelter syndrome), poor breast development in women (Turner Syndrome)

34. Summary of monohybrid crosses C C c C c c C C C What type of monohybrid cross is this an example of?

35. Summary of monohybrid crosses A B O O B A O A O O B O What type of monohybrid cross is this an example of?

36. Summary of monohybrid crosses What type of monohybrid cross is this an example of?