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Mendelian Genetics

Chapter

Lecture Presentation by Dr. Cindy Malone, California State University Northridge

Chapter 3 Learning Objectives

3.1 Understand how Mendel used an experimental model to study patterns of inheritance

3.2 Show how a monohybrid cross reveals how one trait is transmitted from generation to generation

3.3 Explain how Mendel's dihybrid cross generated a unique F2 ratio

3.4 Understand the timeline surrounding Mendel's work

3.5 Learn how independent assortment leads to extensive genetic variations

3.6 To see how math can be used to simplify predictions

3.7 Use statistics (chi-squared analysis) to evaluate the influence of chance on genetic data

3.8 Read pedigrees to reveal patterns of human inheritance

3.9 Understand relationship between a mutant phenotype and a gene sequence

Section 3.1: Mendel’s Experimental Design

Mendel’s model organism: peas

easy to grow

true-breeding strains

controlled matings: self-fertilization or cross-fertilization

grow to maturity in one season

observable characteristics with two distinct forms

Mendel used seven visible features

each with two contrasting traits

true-breeding strains

Mendel kept quantitative records

Figure 3.1

Section 3.1: Mendel’s Legacy

Mendel was not appreciated during his lifetime

His work was rediscovered

Geneticists recognized Mendel had discovered the basis for the transmission of hereditary traits

Section 3.2: Monohybrid Crosses

Monohybrid crosses

True-breeding

Involve a single pair of contrasting traits

P1 generation: Original parents

F1 generation: Offspring

F2 generation: Offspring of F1 generation crossed (self-fertilizing: “Selfing”)

Section 3.2: Constraining Traits

F1 generation monohybrid cross

All plants have just one of two contrasting traits

F2 generation

3/4 of plants exhibit same trait as F1 generation

1/4 exhibit contrasting trait that disappeared in the F1 generation

3:1 ratio (Figure 3.1)

Section 3.2: Reciprocal Crosses

Not sex dependent

F1 and F2 patterns of inheritance similar regardless of P1 source

Dwarf plant pollinates tall plant

Tall plant pollinates dwarf plant

Results for F1 and F2 are the same

Crosses made in both ways  reciprocal crosses

Section 3.2: Particulate Unit Factors

Particulate unit factors (genes)

Basic units of heredity

Are passed unchanged from generation to generation

Determine various traits expressed by each individual plant

Section 3.2: Three of Mendel’s Postulates

Postulate #1: Unit factors exist in pairs

Genetic characters controlled by unit factors

Postulate #2: Dominance/Recessiveness

In a pair of unit factors, one unit is dominant, the other recessive

Postulate #3: Segregation

Paired unit factors segregate (separate) independently during gamete formation

Section 3.2: Genetic Terminology

Phenotype

Physical expression of a trait

Gene

Unit of inheritance

Allele

Alternative form of a single gene

Genotype

Genetic makeup of individual

Alleles written in pairs (DD, Dd, or dd)

Section 3.2: Genetic Terminology (continued)

Organism inherits two alleles

One from each parent

Homozygous/Homozygote

Both alleles are the same (DD, dd)

Heterozygous/Heterozygote

Alleles are different (Dd)

Section 3.2: Punnett Square

Punnett square

Reginald C. Punnett devised this approach

Genotypes and phenotypes resulting from combining gametes can be visualized

Displays all possible random fertilization events

Section 3.2: Testcross: One Character

Testcross

Determines if individual displaying dominant phenotype is homozygous or heterozygous for that trait

Cross between dominant phenotype and homozygous recessive (see Figure 3.4)

Figure 3.4

Section 3.3: Dihybrid Cross

Dihybrid cross

Two pairs of contrasting traits

Generates unique F2 generation (Figure 3.5)

Figure 3.5

Section 3.3: Mendel’s Fourth Postulate

Independent assortment

Unit factors (traits) assort independently during gamete formation

All possible gamete combinations form with equal frequency (Figure 3.6)

Section 3.3: Product Law

Product law

Used to predict frequency of two independent events occurring simultaneously

Example:

F2 plant having yellow and round seeds

3/4  3/4, or 9/16

(Figure 3.6)

Figure 3.6

Section 3.3: Mendel’s 9:3:3:1 Dihybrid Ratio

F1  F1 fertilization event

Each zygote can receive one of four combinations

Example:

9/16 yellow, round seeds

3/16 yellow, wrinkled seeds

3/16 green, round seeds

1/16 green, wrinkled seeds

Gives 9:3:3:1 dihybrid ratio

Figure 3.7

Section 3.3: Testcross: Two Characters

Testcross – Two Characters

Genotypes unknown

Individuals express two dominant traits

Example: Yellow, round seed phenotype in F2 generation

Possible genotypes: GGWW, GGWw, GgWW, GgWw

Section 3.4: Trihybrid Cross

Trihybrid cross

Segregation and independent assortment applied to three pairs of constraining traits

Punnett square with 64 boxes

Easier method:

Use forked-line method (branch diagram) (Figure 3.9)

Figure 3.8

Figure 3.9

Section 3.5: Mendel’s Work Rediscovered

Rediscovered in the early twentieth century

Mendel suggested heredity resulted in discontinuous variation – a dominance- recessive relationship

Darwin and Wallace subscribed to theory of continuous variation: offspring were a blend of parental phenotypes

Section 3.5: Chromosomal Theory of Inheritance

Chromosomal theory of inheritance

Genetic material in living organisms contained in chromosomes

Separation of chromosomes during meiosis served as basis for Mendel’s principles of segregation and independent assortment

Independent assortment leads to extensive genetic variation

Section 3.5: Unit Factors, Genes, and Homologous Chromosomes

Unit factors in pairs

First meiotic prophase

Segregation of unit factors during gamete formation

First meiotic anaphase

Independent assortment of segregating unit factors

Follows many meiotic events

Figure 3.10a

Figure 3.10b

Figure 3.10c

Figure 3.10

Section 3.5: Criteria for Homologous Pairs

Criteria for classifying two chromosomes as homologous pairs

Both are same size and exhibit identical centromere locations

Excludes X and Y chromosomes in mammals

Form pairs or synapse during stages of meiosis

Contain identical linear order of gene loci

Section 3.6: Independent Assortment

Independent assortment leads to extensive genetic variation

Genetic variation is due to nonidentical homologous chromosomes

Chromosome combination produces extensive genetic variation

Section 3.8: Chi-Square Analysis

Chi-square analysis

Evaluates influence of chance on genetic data

Chance deviation

Chance events subject to random fluctuations

Expected outcome is diminished by larger sample size

Section 3.8: Predicting Genetic Outcomes

Two factors in analyzing or predicting genetic outcomes:

Independent assortment

Subject to random fluctuations due to chance deviations

Sample size

Average deviation decreases as sample size increases

Section 3.8: Chi-Square and Null Hypothesis

Null hypothesis

Assumes data will fit given ratio

Assumes there is no real difference between measured values and predicted values

Apparent difference attributed purely to chance

Section 3.8: Chi-Square Analysis

Chi-square (2)

Goodness of fit of null hypothesis

Analysis used to test how well the data fit the null hypothesis

Analysis of observed vs. expected deviations

Table 3.3 shows the steps in 2 calculations for the F2 generation of a monohybrid cross

Table 3.3

Section 3.8: Degrees of Freedom (df)

Degrees of freedom (df)

Equal to n  1

n  number of different categories into which data points may fall (different outcomes)

3:1 ratio: n  2 df  1

9:3:3:1 ratio: n  4 df  3

(Figure 3.11)

Figure 3.11

Section 3.8: Probability Value (p)

When number of degrees of freedom is determined

2 value can be interpreted in terms of a corresponding probability value (p)

Section 3.9: Pedigree

Pedigree

Family tree with respect to given trait

Pedigree analysis reveals patterns of inheritance of human traits

Figure 3.12

Figure 3.13

Section 3.9: Pedigree Analysis

In human traits, extremely valuable tool in human genetic studies (Table 3.4)

Table 3.4

Section 3.10: Mutant Phenotypes Examined at Molecular Level

Mendel’s wrinkled peas: molecular explanation

SBEI: Starch-branching enzyme

Catalyzes formation of branched starch molecules as seed matures

Wrinkled peas lack this enzyme

Osmotic pressure rises  wrinkled peas