1

Patterns of Inheritance

Chapter 13

2

Outline

• Early Ideas of Heredity• Mendel• Gene Disorders• Multiple Alleles• Pedigrees• Gene Disorders Due to Protein Alteration• Chromosome and Inheritance• Genetic Recombination• Human Chromosomes

3

Early Ideas of Heredity

• Classical assumptions– First, heredity occurs within species.

species maintained without significant change since time of creation

– Second, traits are transmitted directly and independently.

paradox - all members of same species should eventually have the same appearance

hybrids differ in appearance

4

Early Ideas of Heredity

• Early geneticists demonstrated some forms of an inherited character can:

– disappear in one generation and reappear, unchanged, in future generations.

– segregate among offspring of a cross.– be more likely to be represented than

alternative forms.

5

Mendel and the Garden Pea

• Advantages of garden pea :– many hybrids previously produced

expect segregation of traits– large number of true-breeding varieties– small and easy to grow

short generation time– sexual organs enclosed in flower

self-fertilization cross fertilization

6

Mendel and the Garden Pea

• Mendel’s experimental design– allowed pea plants to self-fertilize for

several generations assured pure-breeding traits

– performed crosses between varieties exhibiting alternative character forms

– permitted hybrid offspring to self-fertilize for several generations

7

Mendel’s Experiments

8

What Mendel Found

• F1 Generation (first filial)– Offspring of white flower and purple flower

cross had flower color resembling one parent (no intermediate color).

All flowers exhibited purple flowers (dominant trait) and none exhibited white flowers (recessive trait).

9

Mendel’s Results

• F2 Generation (second filial)– Cross between seeds of F1 generation

produced some plants exhibiting white flowers (recessive form reappeared).

dominant : recessive ratio among F2

plants was always close to 3:1Mendelian Ratio

discovered ¼ of recessives were always true breeding

disguised 1:2:1 ratio

10

Second Filial Generation

11

Mendel’s Model of Heredity

• Parents transmit discrete physiological trait information (factors) to offspring.

• Each individual receives two factors that may code for same, or alternative, character traits.

• Not all copies of a factor are identical.– alleles

homozygous - same alleles heterozygous - different alleles

12

Mendel’s Model of Heredity

• Alleles do not influence each other in any way.• Presence of a particular allele does not ensure

its encoded trait will be expressed.– genotype - totality of an individual’s alleles– phenotype - physical appearance

13

Interpretation of Mendel’s Results

• Notational convention– P - dominant allele (purple)– p - recessive allele (white)

PP - homozygous dominant Pp - heterozygous pp - homozygous recessive

14

Interpretation of Mendel’s Results

• F1 generation– PP x pp (parental generation) yielded all

Pp offspring• F2 generation

– Pp x Pp yielded: (1:3:1) ratio 1 PP 3 Pp 1 pp

• Punnett squares

15

Mendel’s Cross

16

Mendelian Inheritance

• Mendel’s First Law of Heredity – (Law of Segregation)

Alternative alleles of a character segregate from each other in heterozygous individuals and remain distinct.

17

Testcross

• Cross of a plant with an unknown genotype (PP or Pp) with a homozygous recessive individual, will yield one of two possible results:

– pp x PP = 100% (Pp)

– pp x Pp = 50% (pp) : 50% (Pp)

18

Testcross

19

Mendelian Inheritance

• Mendel’s Second Law of Heredity – (Law of Independent Assortment)

Genes that are located on different chromosomes assort independently of one another.

20

Mendelian Inheritance

• Phenotype considerations– continuous variation

The greater the number of genes influencing a character, the more continuous the expected distribution of character variation will be.

– pleiotropic effects Individual alleles often have more than

one effect on the phenotype.

21

Phenotypic Considerations

– Incomplete dominance Heterozygotes are intermediate in color.

– Environmental effects degree of allele expression may depend

on the environment– Epistasis

one gene interferes with the expression of another gene

coat color in Labrador retrievers

22

Epistatic Interactions

23

Gene Disorders

• Gene disorder refers to the harmful effect a detrimental allele produces when it occurs at a significant frequency in a population.

– Most disorders are rare because affected individuals often die at a relatively young age, or cannot reproduce.

– Not all defects are recessive. Huntington disease

24

Multiple Alleles: ABO Blood Group

• Codominance - No single allele is dominant, and each allele has its own effect.

– ABO blood groups human gene that encodes enzyme that

adds sugar molecules to lipids on the surface of red blood cells

IB adds galactose IA adds galactosamine i adds no sugar

25

ABO Blood Groups

26

Pedigrees

• Mutations are accidental changes in genes.– rare, random, and usually result in

recessive alleles pedigrees used to study heredity

– hemophilia - inherited condition where blood is slow to clot or does not clot at all

only expressed when individual has no copies of the normal allele

Royal hemophilia - sex-linked

27

Royal Hemophilia Pedigree

28

Gene Disorders Due to Protein Alteration

• Sickle-cell anemia is a recessive inherited disorder in which afflicted individuals have defective hemoglobin, and thus are unable to properly transport oxygen to tissues.

– Homozygotes have sickle-cell.– Heterozygotes usually appear normal, but

are resistant to malaria.

29

Sickle Cell and Malaria

30

Curing Defects with Gene Therapy

• Cystic fibrosis– body cells of affected individuals secrete

thick mucus that clogs airways of lung defect in cf gene

Researchers are currently working on transmitting a working copy of cf gene using viruses.

Early attempts using adenovirus vectors produced mixed results.

31

Chromosomes and Mendelian Inheritance

• In early 20th century, it was not obvious chromosomes were vehicles of heredity information

– chromosomal theory of inheritance first formulated in 1902

problems quickly arose in trying to track independent assortment

32

Chromosomes and Inheritance

• A trait determined by a gene on the sex chromosome is said to be sex-linked.

– In Drosophila, sex is determined by the number of copies of the x chromosome.

Mendelian traits assort independently because chromosomes assort independently.

33

Sex Linkage in Drosophila

34

Genetic Recombination

• Crossing over– Genes located relatively far apart on a

chromosome are more likely to cross over than genes located closer together.

Frequency of crossings can be used to construct a genetic map.

measures distance between genes in terms of recombination frequency

35

Human Chromosomes

• Human somatic cells normally have 23 pairs of chromosomes.

– divided into seven groups characterized by size and shape

– 22 pairs of autosomes– 1 pair of sex chromosomes

XX = Female XY = Male

36

Human Chromosomes

• One x chromosome in females is inactivated early in embryonic development.

– Visible as a darkly staining Barr body attached to the nuclear membrane.

37

Alterations in Chromosome Number

• Failure of chromosomes to separate correctly during meiosis I or II is called primary nondisjunction.

– Down Syndrome caused by trisomy 21 1 in 1700 for mothers < 20. 1 in 1400 for mothers >20<30. 1 in 750 for mothers >30<35. 1 in 16 for mothers >45.

38

Nondisjunction in Sex Chromosomes

• X Chromosome– XXX or XXY yields Klinefelter syndrome– XO yields Turner syndrome

• Y Chromosome– XYY - Jacob syndrome

39

Nondisjunction

40

Genetic Counseling

• Genetic counseling identifies parents at risk of producing children with genetic defects and assesses the state of early embryos.

• High-risk pregnancies– couples with recessive alleles– mothers older than 35

amniocentesis chorionic villi sampling

41

Genetic Counseling

• Counselors can look for three things in cell cultures in search of genetic disorders:

– aneuploidy or gross alterations– proper enzyme functioning– association with known genetic markers

42

Summary

• Early Ideas of Heredity• Mendel• Gene Disorders• Multiple Alleles• Pedigrees• Gene Disorders Due to Protein Alteration• Chromosome and Inheritance• Genetic Recombination• Human Chromosomes

43