Genetic Inheritance According to Gregor Mendel CHAPTER 9• Overview of Inheritance

• Mendel's Pea Plants

• Monohybrid Crosses and Segregation

• Definitions of Genetic Terms

• Dihybrid Crosses and Independent Assortment

• Test Crosses

• The Role of Probability

• Pedigrees and Genetic Conditions

• Human Single-Gene Disorders

• Gregor Mendel

– Was the first person to analyze patterns of inheritance.

– Deduced the fundamental principles of genetics.

Heritable Variation and Patterns of Inheritance

In an Abbey Garden• Mendel studied garden peas

• Pea flowers have both male and female parts

• The ovary of the carpel produces eggs by meiosis

• The anthers of the stamens produce pollen (sperm equivalent) by meiosis

• Pollen grains fly from the anthers of the same flower or from another plant to the carpel and fertilize the eggs in the ovary

• Pea flowers are enclosed within petals such that only insect pollination can effectively transfer pollen from one plant to another

Figure 9.4

How Mendel Fertilized Pea Flower Eggs and Ensured a Single Pollen Source

• He also created true-breeding varieties of plants.

• Mendel then crossed two different true-breeding varieties.

• Mendel performed many experiments.

– He tracked several characteristics in pea plants from which he formulated several hypotheses.

Figure 9.5

True Breeding Varieties Available to Mendel

Figure 9.6a

A Single-Factor (Monohybrid) Cross

• Hypotheses From The Monohybrid Cross

– Some alleles mask or hide the presence of other alleles; these are dominant alleles symbolized by a capital letter (eg. P).

– There are two gene forms (alleles) for every characteristic in the plant

– The letters P and p were used to designate flower color: P = purple, p = white

– There are alternative forms of genes called alleles

– Alleles that can be masked are called recessive alleles, symbolized by a lowercase letter (eg. p)

• Every cell in an organism carries two alleles at a time for every characteristic

– For a pea plant flower color, a plant could have cells with PP, Pp, or pp.

Allelic Combinations: Genotype

• Genotype combinations

– PP is homozygous dominant, yields purple flowers

– pp is homozygous recessive, yields white flowers

– Pp is heterozygous, yields purple flowers

• Phenotype

– An organism’s physical traits

• Genotype

– An organism’s genetic makeup

• Mendel’s law of segregation

– The two members of an allele pair segregate (separate) from each other during the production of gametes.

Some Definitions

Following Mendel’s True Breeding Cross

PP pp

P P P P

Meiosis Meiosis

p p p p Gametes:

P P p pOr more simply:

Next, determine all the possible ways the gametes of one parent can combine with the gametes of another:

Fill out a Punnett Square

P

P

p p

Pp Pp

Pp Pp

Genotype of the offspring are all Pp in the ratio:

4 Pp: 0 PP: 0 pp (Genotypic ratio)

Phenotypic ratio of the offspring:

4 purple: 0 white

True breeding varieties are homozygous

Determining the F2 Offspring

Pp x Pp

Genetic Alleles and Homologous Chromosomes

• Homologous chromosomes

– Have genes at specific loci.

– Have alleles of a gene at the same locus.

Mendel’s Law of Independent Assortment• A dihybrid cross

– Is the mating of parental varieties differing in two characteristics.

• Two hypotheses for gene assortment in a dihybrid cross are

– Dependent assortment.

– Independent assortment

• Mendel’s law of independent assortment states that

– Each pair of alleles segregates independently of the other pairs during gamete formation.

Figure 9.8

Figure 9.23

Figure 9.9

Genes for Coat Color and Vision Sort Independently into Gametes

Using a Testcross to Determine an Unknown Genotype

• A testcross is a mating between

– An individual of unknown genotype and a homozygous recessive individual.

The Rules of Probability• The rule of

multiplication states that

• The probability of a compound event is the product of the separate probabilities of the independent events.

Figure 9.12

Human Characteristics That Follow Mendel’s Laws

• A family pedigree

– Shows the history of a trait in a family.

– Allows geneticists to analyze human traits.

Using a Pedigree to Follow an Autosomal Recessive Gene

Human Disorders Controlled by a Single Gene• Many human traits

– Show simple inheritance patterns.

– Are controlled by genes on autosomes.

Recessive Disorders

• Most human genetic disorders are recessive.

– Individuals can be carriers of these diseases.

Dominant Disorders

• Some human genetic disorders are dominant.

– Achondroplasia is a form of dwarfism.