CHAPTER 10 Mendelian Genetics

1. CHAPTER 10 Mendelian Genetics

2. MENDELIAN GENETICS • Genetics - branch of biology that studies how genetic characteristics are inherited • First investigated by Gregor Mendel, an Augustinian monk (1822-1884) • His work was not accepted until 1900 (30 years later) • The study of inheritance is often called Mendelian genetics.

3. GENE INHERITANCE • Diploid organisms have two forms of a gene – one from each parent • Several different forms (alleles) of each gene may exist within a population • Alleles are found at the same location on a chromosome (called the locus) • Ex: In humans there are two alleles for ear lobe shape (attached and unattached)

4. Ear Lobe Shape

5. GENE INHERITANCE • An organism’s genome is the set of all its genes. • Genotype – genetic makeup of the organism • Phenotype – how alleles are expressed (physical characteristics) • Alleles can be described as: • Dominant – trait that is expressed • Designated by a capital letter (EE) • Recessive – trait is masked by dominant and not expressed • Designated by a lowercase letter (ee)

6. GENE INHERITANCE • Homozygous - alleles are of the same type • both are dominant or both are recessive • AA, aa • Heterozygous – alleles are different • one is dominant and the other is recessive • Aa

7. GENE INHERITANCE • In homozygous individuals, the trait expressed depends on what alleles are present (AA, aa) • Heterozygous individuals are known as carriers: • Only dominant allele is expressed or • Trait will be expressed only under certain conditions • Ex: traits which are sex-linked • The environment also can determine whether or not a trait is expressed • “nature vs. nurture”

8. Dark color in cats is expressed on the parts of the body that stay cool. • The gene for freckles expresses itself more fully when a person is exposed to the sun.

9. Both alleles are expressed (dominant) Ex: petal color in snapdragons Homozygous plants (RR, WW) produce either red or white flowers Heterozygous plants (RW) produce pink colored flowers GENE INHERITANCE:CODOMINANCE

10. GENE INHERITANCE:X-LINKED GENES • Refers to genes found on X-chromosomes • Also called sex-linked genes • These genes are inherited together because they are found on the same chromosome – called a linkage group • Females get two copies of X-linked genes (XX), males get one copy (XY)

11. GENE INHERITANCE:X-LINKED GENES • The Y is much smaller than the X chromosome, thus it has less genes. • In men, because there is only one X, genes on the X-chromosomes will be expressed. • X-linked genes with abnormal traits are: • color blindness • Hemophilia • brown teeth • muscular dystrophy (Becker’s and Duchenne’s) • These are much more common in men

12. SEX CHROMOSOMES

13. MENDEL’S LAW OF HEREDITY • Gregor Mendel developed a method to predict inheritance • His work focused on: • Garden peas - determined dominant and recessive traits • Obvious characteristics (Height, flower color, seed shape, etc.) • One trait at a time • Lead to the development of three laws

14. MENDEL’S LAWS • Law of dominance — an allele that is expressed over another allele is said to be dominant. • Law of segregation— during meiosis, alleles separate from one another into different gametes, retaining their individuality • Law of independent assortment—members of one gene pair separate independently of other gene pairs • Do not apply to linked genes

15. PROBABILITY • Genetic problems are based on probability • The chance that an event will happen • In monohybrid crosses, a single trait is studied. • Crossing tall plants with short plants, green peas with yellow peas, etc. • Punnett squares are tools that are used to determine probability

16. MONOHYBRID CROSSES • A tall pea plant crossed with a tall pea plant will produce offspring that are all tall • TT • T • T • T T • T T • T T • T T

17. MONOHYBRID CROSSES • A short pea plant crossed with a short pea plant will produce offspring that are all short • tt • t • t • t t • t t • t t • t t

18. MONOHYBRID CROSSES • A tall pea plant crossed with a short pea plant will produce all tall offspring: • tt • T • T • Tall is dominant - The phenotype is 100% tall. • The genotype is 100% heterozygous tall (Tt) • T t • T t • T t • T t

19. MONOHYBRID CROSSES • A heterozygous tall pea plant crossed with a heterozygous tall pea plant will produce varying offspring • T t • T • t • One will be homozygous tall, two will be heterozygous tall and one will be homozygous short. • Genotypic ratio - 1:2:1 • Phenotypic ratio - 3:1 (3 tall, 1short) • T T • T t • T t • t t

20. MONOHYBRID CROSSES:CODOMINANCE • A red snapdragon (RR) crossed with a white snapdragon (WW) will produce offspring that are all pink • WW • R • R • All offspring will be heterozygous, and because red and white are codominant, all the flowers will be pink. • Genotypic ratio: all are heterozygous (RW) • Phenotypic ratio: all are pink • RW • RW • RW • RW

21. MONOHYBRID CROSSES: CODOMINANCE • A heterozygous pink crossed with a heterozygous pink, will produce varying offspring: • WR • W • R • One will be homozygous white, two will be heterozygous pink, and one will be homozygous red. • The genotypic ratio will be 1:2:1 and the phenotypic ratio will also be 1:2:1. • WW • RW • RW • RR

22. DIHYBRID CROSSES • Two pairs of alleles are followed from parents to offspring. • Crossing two individuals heterozygous for earlobes and color of hair, the gametes can combine in 16 different ways. • EeDd x EeDd • E – free earlobes; e – attached earlobes; D – dark hair; d – light hair

23. DIHYBRID CROSSES • ED Ed eD ed • ED • Ed • eD • ed • The probability for a given phenotype will be 9:3:3:1. • 9 free earlobes, dark hair • 3 free earlobes, light hair • 3 attached earlobes, dark hair • 1 attached earlobes, light hair • EEDD • EEDd • EeDD • EeDd • EEDd • EEdd • EeDd • Eedd • EeDD • EeDd • eeDD • eeDd • EeDd • Eedd • eeDd • eedd

24. ALTERNATIVE INHERITANCE:MULTIPLE ALLELES • Some genes are expressed by the presence of more than 2 alleles. • Each person only gets two alleles, but in the population, there are more possibilities. • ABO blood type in humans • Determined by 3 alleles (IA, IB, i) • 2 alleles are codominant, one is recessive • GenotypePhenotype • IAIA type A • IAi type A • IBIB type B • IBi type B • IAIB type AB • ii type O

25. ALTERNATIVE INHERITANCE:POLYGENIC INHERITANCE • Traits are determined by several genes at different loci • Genes may be found on the same chromosome or on different chromosomes • Skin color in humans is an example • Dark skin is dominant over light skin • 3 different loci for skin color • Expressed as many possible variations

26. POLYGENIC INHERITANCE

27. ALTERNATIVE INHERITANCE:PLEIOTROPY • Describes the multiple effects that a gene may have on a phenotype • Examples include PKU and Marfan Syndrome • PKU – a single gene affects many chemical reactions (autosomal recessive disorder) • Determines how a cell metabolizes phenylalanine (amino acid) • Causes retardation if not treated with a proper diet • Very light skin color

28. ALTERNATIVE INHERITANCE:PLEIOTROPY • Marfan syndrome affects the connective tissue but can also affect other tissues (autosomal dominant disorder) • Symptoms include: • Disproportionately long arms, legs, and fingers • Skinniness and scoliosis of the spine • Myopia (near-sightedness) and dislocated lenses • Weak or defective heart valves, aneurysms, and aortic dissections • Collapsed lungs and sleep apnea

29. MARFAN SYNDROME

30. ENVIRONMENTAL INFLUENCES • Gene expression may vary on an individual basis • Polydactylism (6 fingers) • Hormones on voice changes at puberty • Male-pattern baldness • PKU expression (proper diet prevents symptoms) • Familial diabetes can be delayed by reducing sugar • Tanning

31. CHAPTER 10 Mendelian Genetics

38. Dark color in cats is expressed on the parts of the body that stay cool. • The gene for freckles expresses itself more fully when a person is exposed to the sun.

46. T T • T T • T T • T T

47. t t • t t • t t • t t

48. T t • T t • T t • T t

49. T T • T t • T t • t t

50. RW • RW • RW • RW