Unit 5C Genetic Inheritance

1. Unit 5C Genetic Inheritance The Work of Gregor Mendel& Applying Mendel’s Principles

2. Heredity and Genetics • Heredity- the delivery of characteristics from parent to offspring • Genetics- the scientific study of heredity • The Modern study of genetics was founded by a monk named Gregor Mendel in the mid- 1800’s • Mendel studied genetics in pea plants

3. Traits • Specific characteristics of an individual • Example: in pea plants- tall or short, round or wrinkled • In humans, brown eyes, blue eyes

4. Crossing Organisms • To study genetic inheritance, Mendel crossed pea plants and looked at how certain traits were passed from generation to generation • To cross means to cause one organism to reproduce with another (like breeding)

5. Mendel Arrived at Two Conclusions • An individual’s characteristics are determined by factors that are passed from one parental generation to the next. • Today, these “factors” are called genes • Each gene has (at least) two contrasting varieties • The different forms of the gene are called alleles • Example: the gene for height in pea plants comes in alleles for tall and short.

6. TT Tttt One dominant and one recessive allele Two recessive alleles Two dominant alleles 2. The Principle of Dominance- some alleles are dominant and some are recessive - Organisms carry two alleles for each trait - Only one dominant allele needs to be present for that trait to be expressed (it “dominates” over the recessive allele) - Two copies of the recessive allele must be present for that trait to be expressed.

7. Why do organisms have two alleles for each gene? • Because we have homologous pairs of chromosomes, one from our mom and one from our dad, we have two copies of every gene.

8. If an allele is dominant does that mean it codes for the more common trait in a population? • Not necessarily! There are many recessive traits that are more common than dominant ones. • If more people in an area have a recessive trait, their offspring will have it too, continuing the trend. • Example: Almost 80% of Northern Europeans have light eyes even though light eyes are recessive to brown eyes.

9. Allele Combinations • Homozygous- organisms with two identical alleles for a particular gene (Ex: TT is homozygous dominant, and tt is homozygous recessive) • Heterozygous- organisms that have two different alleles for the same gene (Tt)

10. Genotype and Phenotype • Genotype- the genetic makeup of an organism (Ex: TT) • Phenotype- the observable characteristic or trait (Ex: Tall) • A plant with the genotype _________ has a phenotype of short. • A plant with the genotype of Tt has a phenotype of ____________.

11. Both possible gametes from Parent 1 T T t Both possible gametes from Parent 2 t Using Punnett Squares • Probability can be used to predict the outcome of genetic crosses. • Example: • Parent 1 is TT • Parent 2 is tt

12. Genotypes: • What percent of the offspring are homozygous dominant? • What percent of the offspring are heterozygous? • What percent of the offspring are homozygous recessive? Phenotypes: • What percent are tall? • What percent are short?

13. When organisms are crossed to determine the inheritance of one gene it is called a monohybrid cross. • The parents used are called the “P” generation and the offspring are called the “F1” (first filial) generation. • Ex: what percent of F1’s will be tall (T) if both parents are heterozygous tall? What is the phenotype ratio of tall (dominant) to short (recessive)?

14. Test Cross • Used to determine if an individual with the dominant trait is homozygous or heterozygous • Example: A pea plant is tall. Is its genotype TT or Tt? • In the lab, cross the tall pea plant with a short pea plant (tt) to see the phenotypes of the offspring. • We can see why this works by setting up both potential situations using punnett squares If 50% of the offspring have the dominant phenotype and the other 50% have recessive, then the parent was Tt If all offspring have the dominant phenotype then the parent was TT

15. Two Factor Crosses • Experiments testing how two genes are passed down are called two-factor or dihybrid crosses. Parent “P” organisms If one parent is homozygous recessive for both genes and the other is homozygous dominant for both genes, then all offspring are heterozygous for both traits R= round r= wrinkled Y=yellow y=green All possible gamete combinations

16. Heterozygous Dihybrid R= round r= wrinkled Y=green y=yellow Determine the possible gametes from each parent and place each combination on the lines outside the box Cross each combination into the boxes with alleles for the same gene coming back together What is the ratio of offspring phenotypes? Both dominantto one dominant one recessiveto one recessive one dominantto both recessive

17. Predictable Phenotypic Ratios for Offspring of Heterozygous Parents • Monohybrid Cross: • Tt X Tt (Two heterozygous parents) • Offspring ratio of dominant phenotype to recessive phenotype will be 3:1 • Dihybrid Cross: • TtGg X TtGg (Two parents heterozygous for both genes) • Offspring phenotypic ratio of both dominant to one dominant, one recessive to one recessive, one dominant to both recessive will be 9:3:3:1

18. Beyond Dominant and Recessive • Not all genes follow the principle of dominance • There are 4 exceptions to the principle of dominance: • Incomplete dominance • Codominance • Multiple alleles • Polygenic traits

19. Incomplete Dominance • Occur when one allele is not completely dominant over the other • Example: four o’clock flowers • Red (RR) x White (WW) = Pink (RW) • Neither red nor white is dominant • Heterozygous phenotypes are a blending of the two homozygous phenotypes

20. Codominance • The phenotypes of both alleles are expressed • Example: In some chickens, black feathers are codominant with white feathers • Heterozygous chickens will have both black and white feathers • Black and white are NOT blended, they appear separately

21. Multiple Alleles • A gene with more than two alleles has “multiple alleles” • Individuals only have two copies of each gene • But, many alleles for the gene can exist in a population • Example: • There are multiple alleles (3) for human blood type: IA, IB, and i. • Alleles IA and IB are codominant • Each person inherits 2 of these alleles, one from mom and one from dad.

22. Multiple Alleles and Codominance Example: Human blood Types Sex-Linked Inheritance Example: Colorblindness Two Different Patterns of Inheritance (besides simple dominance) Key XX= XY= C= c=

23. Polygenic Traits • Traits that are produced by interactions between multiple genes • Examples: At least three genes work together to make the reddish-brown pigment in the eyes of fruit flies • The variety of skin color in humans is because multiple genes interact to produce skin color.

24. Sex-Linked Inheritance Sex-linked genes- genes located on the sex chromosomes (X and Y) Genes on the Y are only found in males and are passed from father to son Genes on the X are found in both sexes, but remember… males have just one X

25. The Consequence of only having one X Chromosome • Recessive disorders related to genes on the X chromosome are more common in males since they only have one X. • Example: color blindness • 3 genes work together to produce color vision and ALL are located on the X chromosome • If any of these alleles is defective in males, they will experience problems seeing colors • In females, if one of these genes is defective, they still may have a good copy of the allele on their other X chromosome!

26. Multiple Alleles and Codominance Example: Sex-Linked Inheritance Example: Two Different Patterns of Inheritance (besides simple dominance) Key XX= XY= C= c= female male Normal color vision Color blind Results:

27. Human Pedigrees • A pedigree is a chart that shows patterns of inheritance in a family.

28. How to read a pedigree • Circles represent females • Squares represent males • Shaded shapes mean the individual expresses the trait • Not shaded shapes mean the individual DOES NOT express the trait • Vertical lines connect parents to offspring • Horizontal lines represent a marriage

29. How to read a pedigree • This pedigree is for the dominant “white forelock” trait • The grandfather expresses the trait • What is the genotype of the circled individual? • What do you think the genotype of the grandfather must be?