patterns of inheritance

PATTERNS OF INHERITANCE

JANUARY 21, 2013

INTERESTING FACT

• All the ova from which the present human poplation was derived can fit into a 5-litre bottle.

• All the sperm that fertilized them could fit in a……

• THIMBLE.

• For years similarities between children and parents obvious.

• Male + female = offspring

• But how?

• Aristotle 384-322 BC semen + menstrual fluid offspring X

• van Leeuwenhook 1677 sperm in semen contain miniature human that implant in female to be born X

• de Graaf 1670s discovered the Graafian follicle from which came the ovum O

• Both parents contribute hereditary characteristics and the offspring is a blend of both.

• The offspring therefore has two sets of genetic information but only one of any pair of genes expresses itself.

MENDEL,1866

THE RUSSIAN MONK

• Mendel performed experiments referring to the process of hybridization in plants.

• He focused on the different forms in which hybrid offspring appear and the statistical relationship between them.

HIS EXPERIMENTS

• He chose the pea Pisum sativum for his research because:

1. Several distinct varieties existed

2. Easy to cultivate

3. Self-pollinating (pure breeding) same characteristics gen. after gen.

4. Artificial cross-breeding varieties possible fertile hybrids.

• Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms:

1. seed shape is (round or wrinkled)2. seed color is (yellow or green)3. flower color is (purple or white)4. pod shape is (inflated or constricted)5. pod color is (yellow or green)6. flower position is (axial or terminal)       7. stem length is (long or short)

HIS SUCCESS

• He succeeded because of careful use of the scientific method.

1. Familiarized self with the plants

2. Only one variable studied at a time

3. Meticulous methodology

4. Sufficient statistical data collected

5. Lack of genetic features such as codominance and linkage

DEFINITIONS• DNA- Deoxyribonucleic acid. This substance

is the back-bone of all genes and therefore assists to make up chromosomes. DNA is found in the nucleus of cells.

• RNA-Ribonucleic acid. This substance helps to make new DNA and hence genes and chromosomes. There are three types, one found in the nucleus and two in the cytoplasm of cells.

• Chromosomes: material found in the nuclei of cells, are composed of DNA and protein and contain genetic information in the form of genes.

• Genes: the basic unit of inheritance for a given characteristic or trait.

• Alleles: contrasting forms of the same gene found occupying the same locus (position) on homologous chromosomes. They may produce the same or different qualities.

• Homozygous: having two identical alleles in corresponding positions on homologous chromosomes.

• Heterozygous: having two contrasting alleles in corresponding positions on homologous chromosomes.

• Phenotype: the outward, visible expression of a gene. E.g. hair colur, eye colour, gender.

• Genotype: the inward, genetic make-up of the organism, especially its alleles (contrasting genes).

• Phenotype: the outward, visible expression of a gene. E.g. hair colour, eye colour, gender.

•

• Dominant: the allele which, if present, shows its effect on the phenotype in both the homozygous and heterozygous conditions. Allele usually represented by capital letters.

• Recessive: the allele which only has an effect on the phenotype of the dominant allele is absent. Allele usually represented by common letters.

• Incomplete Dominance is the expression of alleles that are neither dominant or recessive.

• These alleles mix together and result in expression of a physical trait that is a mixture of the two alleles.

• Example of Incomplete Dominance when you cross a black mice (BB) with a white mice (WW), the progeny of these mice is gray in color (BW). The colors blend together and express the phenotype of both alleles. This type of dominance is similar to mixing paints. When you mix one color with another color, the result is a completely different color. Similarly, when one allele is mixed with another allele they blend together to give rise to a new phenotype.

Incomplete Dominance

• Co-dominance (existing together). • When there are two alleles that share a co-

dominant relationship, progeny will express both alleles.

• For example, if a black cat (CbCb) is crossed with a brown cat (CrCr), the kittens (CbCr) will be either brown with black spots or stripes or black with brown spots or stripes, that is, tabby cat. This means both the colors are co-dominant in this case. Both alleles are completely expressed and the kittens show both colors at the same time.

Codominace

• As you can see, in codominance the alleles express their individual traits simultaneously. In case of incomplete dominance, the alleles tend to blend in to give rise to an altogether new phenotype.

• Examples of Codominance and Incomplete Dominance

Example 1When one crosses a red snapdragon flower (RR) with a white snapdragon flower (WW) the result will be as follows:

RR X WW

will give rise to

RW

The flowers show incomplete dominance as the red and white colors blend and express a completely new color, that is, pink.

• Example 2

When cream-colored mare (CC) is paired with a brown colored horse (BB) it will result in a pony that is tan in color. This is an incomplete dominance example where the alleles blend to give a new phenotype.

CC X BB

this results in an offspring that is tan in color

CB

What would be the phenotypes from CB x CB

• Example # 3 When a red flower is crossed with white flower it results in flowers with red spots on white background or white spots on red background.

R1R1 X R2R2

the resultant flowers are

R1R2

This is an example of codominance where both red and white color are expressed in the resultant flowers.

• Genes, like to play around and express different phenotypes in progeny.

• It is very difficult understanding genes at times.

• Most often we observe complete dominance, where one allele completely takes over another allele.

• Epistasis

MONOHYBRID INHERITANCE

• For monohybrid inheritance Mendel stated that:

• The characteristics of an organism are determined by internal factors which occur in pairs. Only on of a pair of such factors can be represented in a single gamete.

Monohybrid Inheritance

• Inheritance is the process by which certain characteristics or traits are passed on from generation to generation.

• Monohybrid inheritance is the analysis of only one of these traits at a time.

• It is very simple to do.

• A diagram called a Punnett Square is usually used in order to work out monohybrid inheritance.

• We say that we perform a monohybrid cross.

• Definitions to definitely remember when performing these crosses are:

• Dominant• Recessive• Genotype• Phenotype

• Co-dominance and incomplete dominance.

• We will be using monohybrid crossing to predict the genotypic and phenotypic ratios of off-spring born with:

• Different genders (male or female)• Albinism• Sickle-cell• Haemophilia• Night-blindness• Blood types

• Finally, we will be able to trace defects, diseases or traits throughout family trees when we have mastered the monohybrid cross.

• Along the way we will pay attention to ratios that are expected for certain crosses.

Performing Monohybrid Crosses

STEPS• 1). Choose letters to represent the

alleles/genes.• 2). Determine the genotypes of the parents.• 3). Determine the available gametes. • 4). Draw Punnett Square.• 5). Assign gametes.• 6). Fill in Punnett Square.• 7). Analyze results for phenotypic and

genotypic ratios.

Mating for Albinism

• Albino X Albino

• Albino X Homozygous black (normal)

• Albino X Heterozygous black (normal)

• 2 Hetero normals

• Homozygous normal X Hetero normal

• 2 Homozygous normals

Mating for Tallness

• Short X Short

• Short X Homozygous tall

• Short X Heterozygous tall

• 2 Hetero tall

• Homozygous tall X Hetero tall

• 2 Homozygous tall

Eye Colour• 2 Homozygous brown

• 2 Heterozygous brown

• Homo brown X Hetero brown

• Homo brown X Green

• Hetero brown X Green

• 2 Green eyed parents

Inheritance of Gender

• Females have 2 X chromosomes (XX).

• Males have one X and one Y chromosome (XY).

• Let us do a monohybrid cross to determine how sex is passed on from parents to of-spring.

Sex-Linked Genes• Sex-linked genes are genes that are only found

on the longer region of the X chromosome and so will be absent from the Y chromosomes in males.

• When performing crosses involving sex-linked genes, the letters representing the alleles are attached to larger letters that represent the chromosomes, so that we will be able to tell if we are dealing with a man or woman.

• In sex-linkage, women are called carriers, but not men.

Mating for Haemophilia• Normal parents

• Haemophiliac parents

• Haem mom x Haem dad

• Haem mom x normal dad

• Carrier x haemophiliac

• Carrier x normal

Mating for Colour-blindness

• Normal parents

• Colour-blind parents

• Colour blind mom x normal dad

• Colour blind dad x normal mom

• Carrier mom x normal dad

• Carrier mom x colour-blind dad

Co-dominance

• A form of inheritance in which both alleles are equally shown. Blood typing is a great example. AB blood is the codominant relationship between the A protein and B protein both expressing themselves completely. AO (type O allele means there is no protein), A is dominant and you see type A phenotype. BO is the same except you see the B phenotype. Type O is recessive

• With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

Mating for Sickle Cell

• 2 parents with sickle cell disease

• 2 parents with sickle cell trait

• Sickle cell disease X Sickle cell trait

• Sickle cell disease x normal

• Sickle cell trait x normal

Incomplete dominance

• A form of inheritance in which the heterozygous alleles are both expressed, resulting in a combined phenotype. The one example that most books give is seen in some flower colors. A red and a white allele gives pink. If it were codominance, you would see the red and white colors mixed yet separate. Incomplete dominance is most commonly found in plants.

• With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.