The document summarizes key concepts in genetics and inheritance, including: 1) Traits are transmitted by chromosomes which contain genes made of DNA. Genes can be dominant or recessive. 2) Mendel's experiments with pea plants established the basic principles of heredity, including dominance, segregation, and independent assortment. 3) Genes exist in versions called alleles that are received from each parent and can be dominant or recessive.

1. The Genetic Basis of Inheritance From Chromosomes to Genes

2. Chromosomal Basis of Inheritance Traits are transmitted by chromosomes which contain units of heredity called genes Genes are formed from DNA

3. Mendel and the History of Genetics Gregor Mendel Born 1822 Began his work at age 21 A monk and school teacher interested in plant breeding Studied pea plants Discovered the basic principles of heredity

4. Mendel’s Studies Pea plants were an ideal choice for study Displayed seven traits in one of two contrasting forms: seed shape, seed color, seed coat color, pod shape, pod color, flower position, stem length Mendel studied 30,000 plants in 7 years Kept careful records & looked for mathematical patterns

5. Pure Lines Mendel recognized the 2 contrasting forms as distinct varieties, or pure lines In a pure line, the offspring have same trait as the parents ‘ Tall’ always produced ‘tall,’ etc. Called parental pure lines "P1" generation First generation of offspring are F1 (for filial), then F2 and so on Hybrid = An offspring produced by breeding 2 pure lines What happens if you cross 2 pure lines?

6. Dominance Mendel crossed the two pure lines for each trait. These were the P 1 generation. All offspring of these crosses (F 1 ) had the trait of only one parent The trait of the other parent disappeared in the F1 generation (but reappeared in F2) Mendel hypothesized that there were 2 factors for each trait Mendel called 1 factor dominant because it prevailed

7. Dominant vs. Recessive Traits Mendel crossed F 1 plants with other F 1 plants, producing the F 2 generation Not only did the recessive trait reappear in the F 2 generation, but in a consistent proportion: 1/4 showed the recessive trait (ratio 3:1) Mendel referred to the factor that was hidden in the F 1 as recessive If an offspring carries 2 dominant or one dominant and one recessive factor, the offspring will appear to have the dominant trait. If the offspring carries 2 recessive factors, it will appear to have the recessive trait

8. The Experiment P Generation: Cross 2 pure lines – One white, one purple F1 are all purple Purple is dominant Cross F1 x F1 (all purple) ¼ F2 offspring are white White is recessive

9. Principle of Segregation Members of each pair of genes separate, or segregate , when gametes are formed. Recall what we now know happens to chromosomes during meiosis From Mendel: For each characteristic an individual carries 2 ‘factors’ Each parent contributes one of its 2 factors to each offspring. Chances of contributing either factor are equal.

10. Genes and Alleles We now know that the heredity units are genes . The separate forms of a gene that Mendel called ‘factors’ = alleles One allele in a pair may prevent the other from being expressed If an offspring carries 2 dominant, or one dominant and one recessive allele , the offspring will appear to have the dominant trait. If an offspring carries 2 recessive alleles , it will appear to have the recessive trait.

11. Each individual carries one copy (allele) of a gene on the chromosome from their mother, and a second copy on the homologous chromosome from their father.

12. Representing Genes & Alleles Generally the capitalized first letter of the dominant trait is used to represent the dominant allele The recessive allele is then lower case of the same letter (e.g. T for tall, so t for short) Each characteristic can be represented by a pair of letters representing the genes Some multi-allele systems (more than 2 possible forms of the trait) use different conventions Traits carried on sex chromosomes are written as a superscript of the X or Y chromosome

13. Hereditary Terminology Genes – The units of heredity Alleles – The separate forms of a gene that Mendel called factors Genotype - The set of alleles for a characteristic possessed by an organism Phenotype - The appearance of the organism; the trait that is actually expressed Homozygous - The 2 alleles in a pair are identical An individual can be either homozygous dominant (2 identical dominant alleles) or homozygous recessive (2 identical recessive alleles) Heterozygous – The 2 alleles in a pair are different

14. Genotype vs. Phenotype

15. Punnett Squares A way to visualize crosses Punnett square can be used to determine probability of different genotypes or phenotypes Each box contains a possible combination of alleles for offspring Punnett square can be used to determine probability of different genotypes or phenotypes

16. Mendel’s F1 cross can be shown as a punnett square Shows 3:1 ratio

17. Determining Genotype If you know the phenotype, is it possible to determine the genotype? If an organism shows the recessive trait, you know the genotype. It must be homozygous recessive (tt) What if it shows the dominant trait? If the dominant trait is expressed there are 2 possibilities: homozygous dominant ( TT) or heterozygous (Tt)

18. Test Crosses Cross the unknown with a homozygous recessive (tt) x ? This is a test cross (or backcross) If the unknown is TT: then crossing with tt yields all Tt All offspring are tall all have same phenotype and the same genotype If the unknown is genotype Tt, when crossed with tt 1/2 of the offspring will be Tt and 1/2 will be tt so 1/2 will show a different phenotype , making it easy to tell

19. Demonstrating a Test Cross

20. Incomplete Dominance In all of Mendel's cases, 1 allele was clearly dominant This is not always the case Some alleles show incomplete dominance Heterozygotes express traits that are a blend of the phenotypes of the 2 alleles red and white flowers make pink Still obey the law of segregation Only difference is phenotype of homozygous dominant individual is different from phenotype of heterozygous individual

21. Demonstrating Incomplete Dominance

22. Co-Dominance Occurs when both alleles for a gene are expressed in a heterozygote Example – Red & white produce roan color in some animals because both genes are expressed

23. Multi-allele Systems Some traits are the result of more than 2 possible alleles at a locus The ABO blood group is an important example 3 possible alleles: I A - produces A antigen; anti-B antibodies in blood serum I B - produces B antigen; anti-A antibodies in serum i - produces no antigen; anti-A & anti-B antibodies in serum I A & I B are co-dominant alleles i is recessive

24. The ABO Blood System

25. Working with 2 Alleles Crosses so far have examined only 1 characteristic = monohybrid crosses It is possible to consider more than 1 characteristic at a time Crosses involving 2 genes = dihybrid crosses Three genes = trihybrid Mendel crossed plants that differed in 2 characteristics This led to Law of Independent Assortment

26. Law of Independent Assortment Mendel showed that traits produced by dominant factors do not necessarily appear together 2 or more pairs of genes segregate independently of one another during the formation of gametes Genes are distributed to gametes independently We now know this is true: recall independent assortment of chromosomes from meiosis If 2 traits are carried on the same chromosome independent assortment does not hold If 2 traits are located close together on the same chromosome they are linked

27. Dihybrid Crosses Involve 2 genes Can demonstrate independent assortment Cross homzygous dominant for 2 alleles with homozygous recessive for 2 alleles: Y = yellow R = round YYRR x yyrr can produce: Yellow/ round Yellow/ wrinkled Green/ round Green wrinkled Produces 9:3:3:1 ratio

28. Diagramming a Dihybrid Cross

29. Sex Linkage Autosomal trait a gene carried on a non-sex chromosome & present in 2 copies Sex linked trait a gene carried on a sex chromosome that is present in both sexes; one copy in one sex, 2 in the other X chromosome in mammals Z chromosome in birds (one copy in females, 2 in males) Y- linked trait a gene carried on the Y chromosome in humans; very rare

30. Showing Sex Linkage Symbols are written as superscript of the sex chromosome: X a - X chromosome carrying the recessive allele X A – X chromosome carrying the dominant allele No superscript is used for the normal or wild type allele Suspect sex linkage when the ratios of phenotypes are different in males and females

31. Pedigrees Used to determine mode of inheritance when few individuals, but several generations are involved Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait Symbols: O female shaded = affected  male partial shading = carrier

32. Pedigree Analysis

33. Polygenic Inheritance Most traits are not limited to 2 possibilities, yes or no Most traits are a continuum (quantitative) Examples: height, skin color 2 or more genes act additively on a trait

34. Pleitropy Genes have multiple phenotypic effects The same gene that produces dark pigment in skin might produce dark pigment in fur Pleiotropic Advantage: Some genes that have one negative effect, also convey a benefit Example: sickle cell trait may have provided the heterozygote an advantage against malaria

35. Sickle Cell Trait & Pleiotropy

36. Epistasis A gene at one locus alters expression of a second gene Example: If black (B) is dominant over brown (b) in mice, but . . . A second gene (C) determines if fur pigment is made at all, Then you must be (CC) or (Cc) before the mice can be either black or brown

37. Diagramming Epistasis