Patterns of Inheritance

BIO 101

Life Science

Dr. D. L. Daley

Inheritance

The process by which the characteristics of individuals are passed to their offspring

DNA carries the genetic information in the form of sequences of nucleotides

The segments of DNA ranging from hundreds to many thousands of nucleotides are the genes (units) that encode the information that is needed to produce a protein

Chromosomes are made of DNA & proteins

Chromosomes are passed from cell to cell during growth and from organisms to organism during reproduction

Genes are therefore part of chromosomes

Inheritance occurs when genes are transmitted from parent to offspring

Gene Location

A gene’s physical location on a chromosome is called its locus

Each member of a homologous pair of chromosomes (one from each parent) carries the same genes, located at the same loci

Different forms of the gene (nucleotide sequences) at the same locus on the two homologous chromosomes are called alleles

The human ABO blood types have three different alleles of the blood type gene

Alleles May Be The Same Or Different

If both alleles are the same at the given gene locus - the organism is said to be homozygous at the locus

If the two homologous chromosomes have different alleles - the individual is said to be heterozygous - sometimes called a hybrid

Gregory Mendel

Australian monk - father of genetics

Worked with sweet peas

Mendel’s Idea

Proposed - 1860’s

Each parent donates particular heredity factors to each offspring

Before Mendel

Characteristic form each parent blended in a hopelessly complicated manner

Mendel’s Approach

Look at one characteristic at a time

1856 - began to systematically cross-pollinate various true breeding (all offspring have same color colors as parent plant ) varieties of pea plants

Monohybrid cross

Resulting hybrids differ in only one trait

Mendel Found

Monohybrid cross - purple flower with white flower (Parental generation (P))

Only got purple flowers (First generation offspring (F₁)

Mendel’s terminology

Purple flower character - dominant over white flower character which was recessive

What Happens to the Recessive Character?

Disappeared?

To find out Mendel allowed hybrid flowers to become fertilized with their own pollen

929 total plants

705 - purple flowers

224 - white flowers

Thus recessive factors survived within the purple hybrids

Chromosomes in the Gametes of Homozygous & Heterozygous Parents

Gametes for Homozygous Purple and White Flowers

Combinations of Gametes That Produce F₁ Offspring

Combinations of Gametes That Produce F₂ Offspring Flowers

Phenotype versus Genotype

Genotype - actual combination of alleles

PP or Pp

The organism’s physical traits, including its outward appearance, behavior, digestive enzymes, blood type or any other observable or measurable feature make up its phenotype

Punnett Square of a Single Trait

Mendel’s Other Findings

Similar results for six other characteristics

In every case

3/4 of the offspring had dominant character

1/4 had recessive character

3:1 ratio - dominant:recessive

Mendel’s Mathematical Model

(A) denotes dominant character

(a) denotes recessive character

(Aa) denotes hybrid character

Mendel’s Mathematical Model

Each gamete received either (A) or (a) with equal likelihood

Self-fertilizing hybrid

(A) ovum - equal chance of being fertilized by (A) pollen or (a) pollen

Likewise - (a) ovum - equal chance of being fertilized by (A) pollen or (a) pollen

Mendel’s Mathematical Model

All combinations - AA, Aa, aA & aa

Should occur with equal frequency

Aa & aA are identical

Thus - 1:2:1

Since AA & Aa look alike

Ratio of dominate over recessive is 3:1

Mendel’s First Law

Mendel’s Law of Segregation

Mendel’s Model works only if (A) & (a) don’t blend

Thus they remain segregated from each other during formation of gametes

Units of Segregation

Mendel said each plant had a pair of “units” for each character

We know call these genes!

Different forms of a gene, whether dominant or recessive are called alleles

Principle of Segregation

States that the allele determining a dominant character behaves independently of a recessive allele

What About Alleles for Two Characters?

Will round vs. wrinkled seeds still occur in 3:1 ratio whether or not the seed is green or yellow?

If Mendel’s correct

Then - round vs. wrinkled will occur 3:1 regardless of seed color

Math for Two Characters

Proportion of offspring with both characters dominant

3/4 x 3/4 = 9/16

Proportion with one character dominant

3/4 x 1/4 = 3/16

Proportion with recessive characters

1/4 x 1/4 = 1/16

Ratio with 4 combinations of characters

9:3:3:1

Law of Independent Assortment

The independent inheritance of two or more distinct traits is called the law of independent assortment

This law states that the alleles of one gene may be distributed to gametes independently of the alleles of other genes

This occurs when the traits being studied are controlled by genes on different pairs of homologous chromosomes

How are genes located on the same chromosome inherited?

Genes on the same chromosome tend to be inherited together - genetic linkage

One of the first pairs of linked genes to be discovered was found in the sweet pea - different species from Mendel’s peas

In sweet peas, the gene for flower color and the gene for shape are carried on the same chromosome

Thus the alleles would normally assort together into gametes during meiosis or reduction division and are inherited together

Mendel’s Ideas

Mendel’s ideas were quickly accepted

However it soon became clear that most genes affect organisms in a far more complicated way.

Often a given trait is not solely determined by one pair of alleles

Usually many alleles interact with each other and the environment during development and even later in life

Beyond Mendel

Dominance has degrees!

Incomplete dominance - neither allele is fully dominant

Incomplete Dominance: Red and White Four O’clock Example

P₁ generation Red White
(C^R C^R) (C^W C^W)

F₁ generation Pink
(C^R C^W)

F₂ generation Red Pink White
(C^R C^R) (C^R C^W) (C^W C^W)

Ratio = 1:2:1