Mendelian Genetics

CH 11

Question Set 1: Terms of Inheritance

1.What is a homologous chromosome?

2.What is a loci?

3.What is an allele?

4.What is a genotype?

5.What is a gene?

6.What is a trait?

7.How many alleles does a genotype for one trait include?

8.How many alleles does a genotype for two traits include?

9.How many alleles do you get from each parent for a trait?

Inheritance in Modern Terms

• Today, we know that individuals of a species share certain traits because their chromosomes carry the same genes

• The DNA sequence of each gene occurs at a specific location (locus) on a particular chromosome

• locus • Location of a gene on a chromosome

Loci of Some Human Genes

Modern Terms (cont.)

• Diploid cells have pairs of genes, on pairs of homologous chromosomes

• The two genes of a pair may be identical (homozygous), or they may be slightly different alleles (heterozygous)

• homozygous• Having identical alleles of a gene

• heterozygous • Having two different alleles of a gene

Genes on Chromosomes

• Any pair of genes on homologous chromosomes may vary as alleles

• Different alleles may result in different versions of a trait

Modern Terms (cont.)

• The particular set of alleles that an individual carries is their genotype, and their observable traits, such as flower color, make up their phenotype

• genotype • The particular set of alleles carried by an individual

• phenotype• An individual’s observable traits

Modern Terms (cont.)

• Offspring resulting from a cross, or mating, between homozygous individuals with different traits are hybrids

• Often, expression of one allele influences expression of the other, and the outcome is visible in the hybrid phenotype

• hybrid • Offspring of a cross between two individuals that breed

true for different forms of a trait; a heterozygous individual

Modern Terms (cont.)

• An allele is dominant when its effect masks that of a recessive allele paired with it

• dominant • An allele that masks the effect of a recessive allele paired

with it (indicated by italic capital letters: A)

• recessive • An allele whose effect is masked by a dominant allele on

the homologous chromosome (italic lowercase: a)

Question Set 1: Terms of Inheritance

1.What is a homologous chromosome?

2.What is a loci?

3.What is an allele?

4.What is a genotype?

5.What is a gene?

6.What is a trait?

7.How many alleles does a genotype for one trait include?

8.How many alleles does a genotype for two traits include?

9.How many alleles do you get from each parent for a trait?

  Question Set 2: Mendel’s Law of Segregation

1. Why did Mendel work on Pea plants?

2. What is Mendel’s Law of Segregation?

3. During what process does segregation occur?

4. What 2 processes are diagramed when making a Punnett square?

5. What evidence supports the law of segregation?

Key Concepts

• Where Modern Genetics Started• Gregor Mendel gathered evidence of the genetic basis of

inheritance• His meticulous work gave him clues that heritable traits

are specified in units• The units, which are distributed into gametes in

predictable patterns, were later identified as genes

13.2 Mendel, Pea Plants, and Inheritance Patterns

• Gregor Mendel, an Austrian monk, carefully documented how certain traits in pea plants are inherited

Mendel’s Experimental Approach

• Mendel started with garden pea plants that “bred true” for a particular trait, meaning that trait stayed the same in all descendants, generation after generation

• When Mendel cross-fertilized pea plants with different traits, the traits of the offspring appeared in predictable patterns

• Mendel concluded that hereditary information is passed from one generation to the next in discrete units

Fig. 13.2, p. 190

carpel

C

B In this example, pollen from a plant with purple flowers is brushed onto the carpel of a white-flowered plant.

C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant.

A Garden pea flower, cut in half. Male gametes form in pollen grains produced by the anthers, and female gametes form in carpels. Experimenters can control the transfer of hereditary material from one flower to another by snipping off a flower’s anthers (to prevent the flower from self-fertilizing), and then brushing pollen from another flower onto its carpel.

D Every plant that arises from this cross has purple flowers. Predictable patterns such as this offer evidence of how inheritance works.

B

D

A

anther

Breeding Garden Peas

13.3 Mendel’s Law of Segregation

• When Mendel crossed plants that bred true for purple flowers with plants that bred true for white flowers, all of the offspring had purple flowers

• A pea plant with two P alleles (PP) has purple flowers, and one with two p alleles (pp) has white flowers

• The allele for purple (P) is dominant over the allele for white (p), so the heterozygote (Pp) also has purple flowers

Law of Segregation (cont.)

• When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate also

• Each gamete that forms carries only one gene of a pair• Homozygous dominant plants (PP) make (P) gametes • Homozygous recessive plants (pp) make (p) gametes• Heterozygous plants (Pp) make equal numbers of (P) and

(p) gametes

Law of Segregation (cont.)

• When homozygous dominant and homozygous recessive plants are crossed (PP X pp), only one outcome is possible

• A gamete with a P allele meets a gamete with a p allele, and all first generation (F1) offspring will be heterozygous

• Genotype = Pp• Phenotype = purple

Fig. 13.5, p. 192

Stepped Art

gametes (p)

meiosis II

gametes (P)

DNA replication

meiosis I

1 2

zygote (Pp)

3

female gametes

mal

e g

amet

es

4

Gene Segregation

Punnett Squares

• Punnett square • Diagram used to predict the genetic and phenotypic

outcome of a cross

Testcross

• Breeding experiments use testcrosses to determine whether the tested individual is heterozygous or homozygous

• testcross • Method of determining genotype in which an individual of

unknown genotype is crossed with one that is known to be homozygous recessive

Monohybrid Cross

• Another breeding experiment, a monohybrid cross, checks the dominance relationship for a single trait

• monohybrid cross • Breeding experiment in which individuals identically

heterozygous for one gene are crossed• Frequency of traits among offspring offers information

about the dominance relationship between the alleles

A Monohybrid Cross

• In a monohybrid cross between two Pp plants (Pp X Pp), the two types of gametes can meet in four possible ways:

Sperm P meets egg P → zygote genotype PP

Sperm P meets egg p → zygote genotype Pp

Sperm p meets egg P → zygote genotype Pp

Sperm p meets egg p → zygote genotype pp

• The probability that second-generation (F2) offspring will have purple flowers is 3 purple to 1 white, or a ratio of 3:1

PP

Fig. 13.6, p. 193

Stepped Art

Pp

Pp

pp

B A cross between the F1 offspring is a monohybrid cross. The phenotype ratio in F2 offspring in this example is 3:1 (3 purple to 1 white).

P

p

p

P

two types of gametes

Pphybrid

parent planthomozygous

for whiteflowers

parent plant homozygous

for purple flowers

p

pp

P

PP

A All of the F1 offspring of a cross between two plants that breed true for different forms of a trait are identically heterozygous. These offspring make two types of gametes: P and p

A Monohybrid Cross

Traits of Pea Plants

• Mendel found that all these traits of pea plants were inherited in the same pattern in F1 and F2 generations

Mendel’s Law of Segregation

• The 3:1 phenotype ratios in F2 offspring of monohybrid crosses became the basis of Mendel’s law of segregation

• law of segregation • The two members of each pair of genes on homologous

chromosomes end up in different gametes during meiosis

Key Concepts

• Insights From Monohybrid Crosses• During meiosis, pairs of genes on homologous

chromosomes separate and end up in different gametes• Inheritance patterns of alleles associated with different

forms of a trait can be used as evidence of such gene segregation

  Question Set 2: Mendel’s Law of Segregation

1. Why did Mendel work on Pea plants?

2. What is Mendel’s Law of Segregation?

3. During what process does segregation occur?

4. What 2 processes are diagramed when making a Punnett square?

5. What evidence supports the law of segregation?