chapter 10 Mendel and gene idea/ x-linked crosses

Who was Gregor Mendel?

An Austrian monk who discovered the basic principles of heredity through experiments with pea plants and laid the foundation of genetics.

Why did Mendel use pea plants?

They were easy to grow, had many varieties, could self-pollinate or cross-pollinate, and had true-breeding lines.

Why did Mendel work with pea plants?

• Easy to cultivate

• Could self-pollinate or be cross-pollinated

• Had many distinct traits

• Had true-breeding varieties

As a result of Mendel's experiments, what basic principles did he discover?

Law of Segregation and Law of Independent Assortment

Law of Segregation

Each organism has two alleles for a trait. During gamete formation, the alleles separate so each gamete receives only one allele.

Law of Independent Assortment

Alleles of different genes assort independently during gamete formation, allowing different combinations of traits to occur.

Character

A heritable feature, such as flower color.

Trait

A variation of a character, such as purple or white flowers

True-breeding

An organism that always produces offspring with the same trait when self-fertilized. Usually homozygous.

Hybridization

The crossing of two different varieties.

Monohybrid Cross

A genetic cross involving one trait

P Generation

Parental generation

F1 Generation

First filial generation produced from the P generation

F2 Generation

Offspring produced when F1 individuals reproduce

Dominant Trait

A trait expressed whenever the dominant allele is present.

Recessive Trait

A trait only expressed when two recessive alleles are present.

Alleles

Alternative versions of the same gene.

Somatic Cells

Body cells containing two sets of chromosomes (diploid).

Punnett Square

A diagram used to predict offspring genotypes and phenotypes.

Homozygous

Having two identical alleles (AA or aa).

Heterozygous

Having two different alleles (Aa).

Phenotype

The observable appearance of an organism

Genotype

The genetic makeup of an organism.

Conduct a Simple Monohybrid Cross

Write it out

Why is test cross used

A test cross determines whether an organism with a dominant phenotype is homozygous dominant (PP) or heterozygous (Pp).

-If any recessive offspring appear, the unknown parent must be heterozygous.

Conduct a Dihybrid Cross

write it out

Monohybrid Cross (Aa × Aa) Ratio

Genotype:

• 1 AA : 2 Aa : 1 aa

Phenotype:

• 3 Dominant : 1 Recessive

Incomplete Dominance ratio

Genotype:

• 1 : 2 : 1

Phenotype:

• 1 : 2 : 1

Dihybrid Cross (AaBb × AaBb) ratio

Phenotype:

• 9 : 3 : 3 : 1

Genotype:

• 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1

What is Incomplete Dominance?

Incomplete dominance occurs when the heterozygote has an intermediate phenotype between the two homozygotes.

Give an Example of incomplete dominance

• Red snapdragon × White snapdragon

• All F1 offspring are Pink

What is Complete Dominance?

The phenotype of a heterozygote is identical to the phenotype of the dominant homozygote.

What is Codominance?

Codominance occurs when both alleles are fully expressed in the heterozygote.

give an example of codominance

Human AB blood type (IAIB)

Multiple Alleles

A gene with more than two alleles in a population.

Multiple Alleles

A gene with more than two alleles in a population.

Example: ABO blood types (IA, IB, i).

Pleiotropy

One gene affects multiple traits.

ex) sickle cell

Epistasis

One gene affects or masks the expression of another gene

ex) mouse coat color inheritance

Polygenic Inheritance

Polygenic Inheritance

ex) skin color

What is the Chromosomal Theory of Inheritance?

Genes are located on chromosomes, and chromosomes undergo segregation and independent assortment during meiosis.

X-Linked Genes

Genes located on the X chromosome

Linked Genes

Genes located on the same chromosome and therefore tend to be inherited together

What is a Gene Locus?

A gene locus is the specific location of a gene on a chromosome.

What are the Sex Chromosomes for Humans?

Humans:

• Female = XX

• Male = XY

Give Examples of Sex-Linked Genes.

• Color blindness

• Hemophilia

• Fruit fly eye color gene

What are Barr Bodies in Female Mammals?

Barr bodies are inactive X chromosomes found in female mammal cells. During embryonic development, one X chromosome becomes inactive.

Nondisjunction

Failure of chromosomes to separate properly during meiosis.

Disjunction

Normal separation of chromosomes during meiosis.

Euploidy

Normal chromosome number.

Aneuploidy

Abnormal chromosome number caused by chromosome gain or loss

Trisomy

Three copies of a chromosome.

Monosomy

One copy of a chromosome.

Polyploidy

More than two complete sets of chromosomes. Common in plants

What is a Karyotype?

A karyotype is an organized picture of an individual's chromosomes arranged in homologous pairs by size, shape, and number. It is used to identify chromosomal abnormalities such as Down syndrome and Turner syndrome.