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Patterns of inheritance

Inheritance: monohybrid, dihybrid and co-dominance

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Explainer Video

Tutor: Jasmine

Summary

Inheritance: monohybrid, dihybrid and co-dominance

In a nutshell

Inheritance is the process in which genes are given to the offspring from parents. Is it possible to predict the phenotype of offspring using the parental genotypes and Punnet squares. Scientists then compare their observed results with the results expected using a chi-squared test.

Definitions

Key word	Definition	Example
Gene	The sequences of bases in a DNA molecule that encode for a protein that results in a characteristic.	A gene that encodes for pea pod colour.
Allele	Different versions of a gene.	Pea pods can be yellow or green.
Genotype	The genetic makeup of an organism.	Peas can be GG, Gg or gg for pod colour.
Phenotype	The expression of an organism's genetic makeup and the interaction it has with the environment.	Green pods.
Dominant	An allele whose characteristic always appears in the phenotype if it is present.	The allele for green pods (G) is dominant so having the genotype GG or Gg will produce green pods.
Recessive	An allele whose characteristic only appears when two copies are present.	The allele for yellow pods (g) is recessive, so if the pea has the genotype gg it will be yellow.
Codominant	Alleles that are both expressed in the phenotype.	The alleles for haemoglobin are codominant.
Locus	A fixed position on a chromosome where a specific gene is located.	The locus for pea pod colour is located on chromosome $5$ in peas.
Homozygote	An organism that carried two copies of the same allele.	GG or gg.
Heterozygote	An organism that carries two different alleles.	Gg.
Carrier	A person who carries an allele that is not expressed in their phenotype but can be passed on to their offspring.	A person may be a carrier of cystic fibrosis but not have the disease.

Monohybrid inheritance

Humans are diploid organisms, this means we have two sets of chromosomes and two alleles for each gene. One is inherited from the mother and the other is inherited from the father.

Monohybrid inheritance is the inheritance of a characteristic that is controlled by a single gene. We use monohybrid crosses to show the likelihood of different alleles of that gene being inherited.

Example

Work out the genotypes and phenotypes of the offsprings when one homozygous green pea and one yellow pea are bred.

Biology; Origins of genetic variation; KS5 Year 12; Inheritance: monohybrid, dihybrid and co-dominance

Therefore each offspring will have green pods.

Procedure

1.	Write out the alleles.
2.	Write out the parental genotypes
3.	Draw the gametes.
4.	Draw punnet square.
5.	State the phenotypes of each genotype.

These peas are pure bred, this means they are homozygous for a particular gene. When pure breeding plants are crossed this is known as the F₁ generation. When the offspring from an F₁ generation are crossed, this is known as the F₂ generation.

Example

Work out the genotypes and phenotypes when two F₁ peas are crossed.

Therefore the phenotypic ratio of green pods to yellow is $3:1$ .

Dihybrid inheritance

Definition

This is the inheritance of two different genes located on different chromosomes.

Example

Two pure breeding plants were crossed. One always produces round, yellow seeds (dominant features) and the other always produces wrinkled, green seeds (recessive features). Work out the genotypes and phenotypes of the offspring.

Therefore all of the offspring are heterozygous.

Example

Work out the genotypes and phenotypes of the F₂ generation.

$9$ seeds will be yellow and round, $3$ will be green and round, $3$ will be yellow and wrinkled and $1$ will be green and wrinkled. Therefore, the phenotypic ratio will be $9:3:3:1$ . 

Codominance

This is when neither alleles are recessive so both are expressed.

Example

Sickle cell anaemia

1.	People who are homozygous for normal haemoglobin (H^NH^N), do not have the disease.
2.	People who are homozygous for sickle haemoglobin (H^SH^S), have the disease.
3.	People who are heterozygous (H^NH^S) have sickle-cell trait, this is the in-between phenotype. They have some normal shaped haemoglobin and some sickle haemoglobin.

Multiple alleles

Some genes have more than two alleles

Example

The ABO blood group system has three alleles for blood type in humans.

Allele	Blood group	Type of allele
I^O	O	Recessive
I^A	A	Codominant
I^B	B	Codominant

This means people with the genotype I^AI^B will have the blood group AB.

Sex-linkage

Any gene that is carried on the X or Y chromosome is referred to as sex-linked. The Y chromosome is smaller and therefore carries fewer genes, this means most genes are carried by the X chromosome. Males only have one X chromosome so they are more likely to express recessive phenotypes. Genetic disorders caused by faulty alleles on sex chromosomes impact males at a greater rate for this reason.

Example

Haemophilia

Autosomal linkage

Autosomes are chromosomes that aren't sex chromosomes, genes on the same autosome are linked. This is because they'll stay together during independent segregation of chromosomes during meiosis I and their alleles will be passed on to offspring together.

The closer together two genes are on an autosome, the more closely linked they are. This is because they are less likely to be split up during crossing over.

Epistasis

Definition

When the allele of one gene affects or masks the expression of another in a phenotype.

Example

In mice, several genes control the expression of fur colour. Gene A controls the expression of a black pigment called melanin. The dominant allele produces bands of black hair, whereas the recessive genotype (aa) produces uniform black fur. Gene B controls the fur colour by determining the expression of gene A. The presence of the dominant allele leads to the production of melanin, the absence of the dominant allele (bb) leads to no melanin production and white fur (albino). This means regardless of the alleles of gene A, the mouse will always be white if it has the genotype bb.

Chi-squared test

The chi-squared test is used to test the null hypothesis. The null hypothesis is used to examine the results of scientific experiments. It is based on the assumption that there is no statistically significant difference between the set of observations and any differences are due solely to chance.

$chi\ squared= sum\ of {(observed\ numbers - expected\ numbers)^2\over expected\ numbers}$

$\chi^2 = \sum{(O-E)^2\over E}$

Procedure

1.	Work out the chi-squared value using the above formula.
2.	Compare your value to the critical value. This is the value of chi-squared that corresponds to a $0.05\ (5\%)$ level of probability that the difference between the observed and expected values is due to chance.
3.	If $\chi^2$ is smaller than the critical value then there is no significant difference between observed and expected results. The null hypothesis cannot be rejected.
4.	If $\chi^2$ is larger than or equal to the critical value, then there is a significant difference between the observed and expected results. The null hypothesis can be rejected and something other than chance is causing the difference.

Learn with Basics

Length:

Unit 1

DNA structure, discovery and inheritance

Unit 2

Inheritance and genetic diagrams

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Unit 3