Patterns of Inheritance

Explain the Laws of Segregation and of Independent Assortment and what features of meiosis they correspond to.

Explain the terms gene, allele, dominant, recessive, genotype, phenotype, heterozygous, homozygous,

Solve genetics problems involving monohybrid and dihybrid crosses.

Explain incomplete dominance and co-dominance.

Learning Outcomes

Mendelian Genetics

Gregor Mendel 1822-1884

“father of genetics”

Proposed that characteristics were passed from generation to generation by “heritable factors”

Mendelian Genetics

Mendel investigated many characteristics of garden peas

Stamen removal Pollen transfer

Carpel matures into pod

Seeds from pod planted

Offspring (F1)

Parents (P) Carpel

Stamens

1 2

3

4

Monohybrid Cross

True Breeding Parents

F1 generation

Monohybrid Cross

True Breeding Parents

F1 generation

all purple

Monohybrid Cross

F1 generation

F2 generation

3 purple : 1 white

Mendel studied 7 traits. From his studies Mendel developed several ideas about the inheritance of traits.

Mendel’s Conclusions

A plant has two heritable factors for each character (e.g., flower color). When a plant makes gametes, only one heritable factor for each trait is given to each gamete. When gametes unite, the new plant will have a heritable factor from each parent. Some heritable factors dominate over others.

The two copies of a particular gene separate when an individual makes gametes.

Each gamete receives only one copy.

Mendel’s Law of Segregation

Dihybrid Cross

True Breeding Parents

F1 generation all TALL with Purple flowers

F2 generation

F1 generation

Dihybrid Cross

PpTt PpTt x

9 : 3 : 3 : 1

The law of independent assortment. Alleles of different genes assort

independently during gamete formation. This law applies to genes on different

chromosomes.

Mendel’s Law of Independent Assortment

Terminology

Allele – one version of the gene for a particular character

many genes have multiple alleles

Genotype – the two specific alleles that an individual has

--the genetic makeup of the individual

Homozygous – the two alleles are the same

Heterozygous – the two alleles are different

Phenotype – the appearance of the individual

Dominant – presence of the allele will confer the phenotype SYMBOL USES CAPITAL LETTER

Recessive – the phenotype will only be seen when both alleles are recessive SYMBOL USES lowercase LETTER

Co-dominant – both alleles contribute to phenotype

Terminology

How do Mendel’s Laws apply to Meiosis?

zygote (Pp)

gametes (p)

meiosis II

gametes (P)

DNA replication

meiosis I

1 2

3

meiosis II

meiosis I

meiosis II

meiosis I

or

Pt pT PT pt

Mendel’s Laws

Law of Independent Assortment heritable factors for one trait segregate independently of the heritable factors for other traits

Law of Segregation each gamete contains only one heritable factor for each trait

Homologous chromosomes separate at meiosis I

The orientation of each tetrad at the metaphase plate in Meiosis I is random (i.e., independent from orientation of other tetrads.)

Alleles occupy a physical position on a chromosome

The Law of Independent Assortment applies to genes on different chromosomes.

Genes on the same chromosome tend to be inherited together

Incomplete Dominance

F 1

F 2

Phenotypes

Mild disease Severe disease Normal Cell

LDL

LDL receptor

Genotypes HH

Homozygous for ability to make

LDL receptors

Hh Heterozygous

hh Homozygous

for inability to make LDL receptors

Incomplete Dominance

Two alleles of a gene produce phenotypes that are both present in the heterozygote.

Example: ABO blood group system has three

alleles of the gene: IA, IB, and IO.

Codominance

O A B

Codominance

The A, B, O sugars are covalently linked to phospholipid headgroups on the RBC plasma membrane.

Why do we care?

Carbohydrate A

Carbohydrate B

Carbohydrate A and Carbohydrate B

Neither

Blood Group (Phenotype) Genotypes

Carbohydrates Present on Red Blood Cells

Antibodies Present in Blood

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

A O B AB

Anti-B

Anti-A

None

Anti-A

No reaction Clumping reaction

Anti-B

A

O

B

AB

IAIA or IAi

IBIB or IBi

IAIB

ii

Blood Group Effects on Health

Type O compared to Type A, AB, and B individuals have a 14% reduced risk of squamous cell carcinoma and 4% reduced risk of basal cell carcinoma. a reduced risk of pancreatic cancer. Type B correlated with increased risk of ovarian cancer, Gastric cancer reported to be most common in blood group AB and least in group O.

Pleiotropy – one gene having many effects on phenotype.

Epistasis – the protein product of one gene controls the expression of a second gene.

More about these in Chapter 14