PATTERNS OF INHERITANCECAPTER 10

Pre-Mendel’s Theories

Blending Hypothesis: when parents with different traits have offspring, this will always show a blending of the traits

Spontaneous generation: Non-living matter giving rise to living matter

Homunculus: The little man inside the sperm

Trait: a variation of a particular character

Particulate hypothesis: Parents pass on to their offspring separate and distinct factors (genes) that are responsible for inherited traits

Genetics: the study of heredity

Mendel’s experiments with pea plants

Mendel’s experiments started with

True-breeding plants: when self-fertilized, a true-breeding plant produces offspring identical in appearance to itself generation after generation

Cross-fertilization, sperm from the pollen of one flower fertilizes the eggs in the flower of a different plant

the offspring of two different true-breeding varieties are called hybrids.

Monohybrid cross: pairing in which the parent

plants differ in only one (mono) character.

Mendel’s principle of segregation

There are alternative forms of factors (genes) called alleles.

For each character, an organism has two alleles for the gene controlling that character, one from each parent. Homozygous = same alleles Heterozygous = different alleles

Principle of segregation (cont.)

When only one of the two different alleles in an heterozygous individual appears to affect the trait, that allele is called the dominant allele. The allele that does not appear to affect the trait is called the recessive allele

The two alleles for a character segregate (separate) during the formation of gametes (sex cells). Each gamete carries only one allele of each character (Principle of segregation)

Phenotype refers to the observable trait (purple flowers)

Genotype refers to the combination of alleles (PP)Phenotypic ratio: ratio of plants with purple flowers to those with white flowers (3 purple : 1 white) Genotypic ratio: ratio of possible combinations of alleles (1 PP : 2 Pp : 1 pp)

Probability and Punnett Squares

Probability: Chance of Something happening

Punnett Square: Diagramused to show the probabilityof a genetic cross

Phenotype refers to the observable trait (purple flowers)

Genotype refers to the combination of alleles (Pp)

• Phenotypic ratio: ratio of plants with purple flowers to those with white flowers (3 purple : 1 white)

• Genotypic ratio: ratio of possible combinations of alleles (1 PP : 2 Pp : 1 pp)

How can you find out the genotype of an individual showing the dominant trait?

TESTCROSS

In a testcross, an individual of unknown genotype, but dominant phenotype is crossed with a homozygous recessive individual

DIHYBRID CROSS

crossing of organisms differing in two characters

Principle of Independent Assortment

During gamete formation in an F2 cross, a particular allele for one character can be paired with either allele of another character

The alleles for different genes are sorted into the gametes independently of one another.

Not all traits are inherited following the patterns found by Mendel in pea plants

Intermediate or incomplete inheritance

Multiple alleles – codominance

Polygenic inheritance

Environment - epigenetics

Intermediate or incomplete inheritance

The heterozygotes have a phenotype that is intermediate between the phenotypes of the two homozygotes

Multiple alleles - codominance

For many genes several alleles exist in the population.

Multiple alleles control the character of blood type in humans.

There are six possible genotypes.

The alleles IA and IB exhibit codominance, meaning that a heterozygote expresses both traits.

http://www.youtube.com/watch?v=oz4Ctau8mC8

Polygenic inheritance

When two or more genes affect a single character

In humans, height and skin color have polygenic inheritance

Chromosome Theory of Inheritance

Genes are located on chromosomes

Behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns – (chromosomes undergo segregation and independent assortment during meiosis)

Gene locus: location at which alleles of a gene reside on homologous chromosomes

Linked genes: genes that are located in the same region of a chromosome

Genetic linkage: tendency for the alleles on one chromosome to be inherited together. The closer two genes are on a chromosome, the greater the genetic linkage

Sex-Linked Genes (any gene located on sex chromosomes)

http://www.youtube.com/watch?v=vP_h08cT5jw

Morgan's monohybrid cross for fly eye color produced a 3 : 1 phenotypic ratio of red to white eyes in the F2 generation. However, none of the flies with white eyes were female.