Chapter 11. Classical (Mendelian) Genetics

Transcription

1 Chapter 11 Classical (Mendelian) Genetics

2 The study of how genes bring about characteristics, or traits, in living things and how those characteristics are inherited. Genetics

3 Geneticist A scientist who studies genetics

4 Portions of DNA molecules that determine characteristics of living things Genes

5 Through the processes of meiosis and reproduction, genes are transmitted from one generation to the next

6 Augustinian monk Developed the science of genetics Performed is experiments in the 1860 s and 1870 s, although the scientific community did not accept his work until early in the twentieth century Gregor Mendel

7 Because the principles established by Mendel form the basis for genetics, the science is often referred to as Mendelian genetics It is also called classical genetics to distinguish it from another branch of biology known as molecular genetics

8 Mendel believed that factors pass from parents to their offspring, but he did not know of the existence of DNA Modern scientists accept that genes are composed of segments of DNA molecules that control discrete hereditary characteristics

9 Diploid Most complex organisms have cells that are diploid Diploid cells have a double set of chromosomes, one from each parent

10 Human cells have a double set of chromosomes consisting of 23 pairs, or a total of 46 chromosomes In a diploid cell, there are two genes for each characteristic Example

11 In preparation for sexual reproduction, the diploid number of chromosomes is reduced to a haploid number Diploid cells are reduced to cells that have a single set of chromosomes Haploid

12 These haploid cells are gametes, or sex cells, and they are formed through meiosis When gametes come together in sexual reproduction, the diploid condition is reestablished Gametes

13 Alleles The offspring of sexual reproduction obtain one gene of each type from each parent The different forms of a gene are called alleles

14 In humans, there are two alleles for earlobe construction One allele is for earlobes that are attached while the other allele is for earlobes that hang free The type of earlobe a person has is determined by the alleles inherited from the parents Example

15 Genome The set of all genes that specify an organism s traits is known as the organism s genome The genome for a human cell consists of about 100,000 genes

16 The gene composition of a living organism is its genotype For a person s earlobe shape, the genotype may consist of two genes for attached earlobes, or two genes for free earlobes, or one gene for attached and one gene for free earlobes Genotype

17 Phenotype The expression of the genes is referred to as the phenotype of a living thing If a person has attached earlobes, the phenotype is attached earlobes If the person has free earlobes, the phenotype is free earlobes

18 Even though three genotypes for earlobe shape are possible, only two phenotypes (attached earlobes and free earlobes) are possible

19 A phenotype can, in some instances, be affected by the environment The dark ears of a Himalayan rabbit show up as a result of their cool temperature in relation to the rest of the rabbit s body

20 Genotype The two paired alleles in an organism s genotype may be identical, or they may be different

21 Two identical alleles are present for a particular characteristic Homozygous

22 Two different alleles are present for a particular characteristic Heterozygous

23 In a homozygous individual, the alleles express themselves In a heterozygous individual, the alleles may interact with one another, and in many cases, only one allele is expressed

24 When one allele expresses itself and the other does not, the one expressing itself is the dominant allele The overshadowed allele is the recessive allele

25 In humans, the allele for free earlobes is the dominant allele If this allele is present with the allele for attached earlobes, the allele for free earlobes expresses itself, and the phenotype of the individual is free earlobes

26 Dominant alleles always express themselves, while recessive alleles express themselves only when two recessive alleles exist together in an individual

27 A person having free earlobes can have one dominant allele or two dominant alleles, while a person having attached earlobes must have two recessive alleles

28 Inheritance Patterns Mendel was the first scientist to develop a method for predicting the outcome of inheritance patterns

29 He performed his work with pea plants, studying seven traits: plant height, pod shape, pod color, seed shape, seed color, flower color and flower location

30 Pea plants pollinate themselves Over many generations, pea plants develop individuals that are homozygous for particular characteristics These populations are known are pure lines or purebred

31 In his work, Mendel took pure-line pea plants and crosspollinated them with other pure-line pea plants He called these plants the parent generation

32 When Mendel crossed pure-line tall plants with pure-line short plants, he discovered that all the plants resulting from this cross were tall He called this generation the F 1 generation (first filial generation)

33 Next, Mendel crossed the offspring of the F 1 generation tall plants among themselves to produce a new generation called the F 2 generation (second filial generation)

34 Among the plants in this generation, Mendel observed that 3/4 of the plants were tall and 1/4 of the plants were short

35 Mendel s laws of genetics Mendel conducted similar experiments with the other pea plant traits Over many years, he formulated several principles that are known today as Mendel s laws of genetics

36 Mendel s laws of dominance When an organism has two different alleles for a trait, one allele dominates Tall:Short

37 Mendel s law of segregation During gamete formation by a diploid organism, the pair of alleles for a particular trait separate, or segregate, during the formation of gametes (as in meiosis)

38 Mendel s law of independent The members of a gene pair separate from one another independent of the members of the other gene pairs (These separations occur in the formation of gametes during meiosis) assortment

39 Mendelian crosses An advantage of genetics is that scientists can predict the probability of inherited traits in offspring by performing a genetic cross (also called a Mendelian cross) To predict the possibility of an individual trait, several steps are followed

40 Steps to Solving Genetics Problems 1. Choose a letter to represent the genes in the cross

41 2. Write the genotypes of the parents

42 3. Determine the possible gametes that the parents can produce

43 4. Enter the possible gametes at the top & side of the Punnett square

44 5. Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes

45 6. Determine the genotypic & phenotypic ratios of the offspring

46 7. Using information from #5 & #6, answer the problem

47 A Punnett square is a boxed figure used to determine the probability of genotypes and phenotypes in the offspring of a genetic cross Punnett square

48 The possible gametes produced by the female are indicated at the top of the square, while the possible gametes produced by the male are indicated at the left side of the square Punnett square