INHERITANCE HL
Nature of science:
Looking for patterns, trends and discrepancies—Mendel used observations of the natural world to find and explain patterns and trends. Since then, scientists have looked for discrepancies and asked questions based on further observations to show exceptions to the rules. For example, Morgan discovered non-Mendelian ratios in his experiments with Drosophila. (3.1)
dyhibrid crsses
Consider two traits, each carried on separate chromsomes (the genes are unlinked).
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous at both loci?
In this example of Lathyrus odoratus (sweet pea),
we consider two traits: pea colour and pea surface.
What is the predicted phenotype ratio for a cross between two pea plants which are heterozygous at both loci?
Autosomes and sex chromosomes
In humans the sex chromosomes are the X and the Y, and they are the ones that determine whether a person is female or male. Any chromosome that is not a sex chromosome is called and autosome. Humans have 22 pairs of autosomes and one pair of sex chromosomes.
If a trait or gene is described as autosomal, its locus is on one of the 22 pairs of autosomes, a trait or that is said to be sex-linked must have its locus on a sex chromosome. Where a gene is located determines whether or not the trait it controls is more common in males or females. When a trait is more common in one sex than the other, the chances are that the trait is sex-linked and that the locus is on the X or Y chromosomes.
If a trait or gene is described as autosomal, its locus is on one of the 22 pairs of autosomes, a trait or that is said to be sex-linked must have its locus on a sex chromosome. Where a gene is located determines whether or not the trait it controls is more common in males or females. When a trait is more common in one sex than the other, the chances are that the trait is sex-linked and that the locus is on the X or Y chromosomes.
Linkage groups
Are carried on the same chromosome and are inherited together. They do not assort independently.
Any two genes that are found on the same chromosome are said to belinked to each other. Liked genes are usually passed on to the next generation together.
As group of genes inherited together because they are found on the same chromosome are considered to be members of a linkage group. This applies to genes found on autosomes as well as those on the sex chromosomes.
Any two genes that are found on the same chromosome are said to belinked to each other. Liked genes are usually passed on to the next generation together.
As group of genes inherited together because they are found on the same chromosome are considered to be members of a linkage group. This applies to genes found on autosomes as well as those on the sex chromosomes.
linked genes
Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome
EX: The gene for bod color is in the same linkage group as the gene for the wing length, this means that the loci for these two genes are located on the same chromosome and are therefore considered to be linked genes
Linked genes are pairs or groups of genes which are inherited together, carried on the same chromosome
EX: The gene for bod color is in the same linkage group as the gene for the wing length, this means that the loci for these two genes are located on the same chromosome and are therefore considered to be linked genes
gene linkage and Recombination
Of alleles occurs as a result of crossing-over between non-sister chromatids. Exchange of alleles gives new genotypes of gametes.
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened
Plants which are heterozygous at both loci are test-crossed.
A small number of purple;short and white;long individuals have appeared in the offspring. Explain what has happened
The further apart a pair of alleles are on a chromosome, the more likely it is that crossing over may occur between them - leading to recombination.
Knowing this, researchers can map the position of genes on a chromosome based on the frequency of recombination between gene pairs: the further apart they are, the more often they cross over.
Knowing this, researchers can map the position of genes on a chromosome based on the frequency of recombination between gene pairs: the further apart they are, the more often they cross over.
polygenic inheritance
A single characteristic controlled by multiple genes.
Polygenic inheritance gives rise to continuous variation in the phenotype.
Involves 2 or more genes influencing the expression of one trait. With two or more allelic pairs found at different loci, the number of possible genotypes is greatly increased. It is believed that human traits are too complex to show all combinations.
Polygenic inheritance gives rise to continuous variation in the phenotype.
Involves 2 or more genes influencing the expression of one trait. With two or more allelic pairs found at different loci, the number of possible genotypes is greatly increased. It is believed that human traits are too complex to show all combinations.
Continuous and discrete variation
With dominant and recessive alleles of a single gene, the number of possible phenotypes is limited. For example a person either has cystic fibrosis or not. When mutlitples alleles are introduced the number of possibilities increases. When a second gene is introduced the number of possible genotypes increases dramatically. And so on. In humans continuous variation also can be seen in the genetic components of traits such as height, body shape, intellectual. Each of these is also influenced by environmental components.
multiple alleles
So far, only two possibilities have been considered for a gene: dominant, A, or recessive, a. With two alleles, three different genotypes are possible, which can produce two different phenotypes. However, genetics is not always this simple; sometimes there are three or more alleles for the same gene. This is the case for the alleles that determine the ABO blood type in humans.
Blood type: an example of multiple alleles
The ABO blood type system in humans has four possible phenotypes: A, B, AB and O. To create these four blood types there are three alleles of the gene. These three alleles can produce six different genotypes.
The gene for the ABO blood type is represented by the letter I. To represent more than just two alleles (I and i) superscripts are introduced. As a result, the three alleles for blood type are written as follows: IA, IB and i. The two capital letters with superscripts represent alleles that are co-dominant:
• IA = the allele for producing proteins called type A antigens, giving type A blood
• IB = the allele for producing proteins called type B antigens, giving type B blood
• i = the recessive allele that produces neither A nor B antigens, giving type O blood.
Crossing these together in all possible combinations creates six genotypes that give rise to the four phenotypes listed earlier:
• IAIA or IAi gives a phenotype of type A blood
• IBIB or IBi gives type B blood
• IAIB gives type AB blood (because of co-dominance, both types of antigens are
produced)
• ii gives type O blood.
Notice how the genotype IAIB clearly shows co-dominance. Neither allele is masked: both are expressed in the phenotype of type AB blood.
Applications and skills:
• Application: Morgan’s discovery of non-Mendelian ratios in Drosophila.
• Application: Completion and analysis of Punnett squares for dihybrid traits.
• Application: Polygenic traits such as human height may also be influenced by environmental factors.
• Skill: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
• Skill: Identification of recombinants in crosses involving two linked genes. • Skill: Use of a chi-squared test on data from dihybrid crosses.
Theory of knowledge:
• The law of independent assortment was soon found to have exceptions when looking at linked genes. What is the difference between a law and a theory in science?
• Application: Morgan’s discovery of non-Mendelian ratios in Drosophila.
• Application: Completion and analysis of Punnett squares for dihybrid traits.
• Application: Polygenic traits such as human height may also be influenced by environmental factors.
• Skill: Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
• Skill: Identification of recombinants in crosses involving two linked genes. • Skill: Use of a chi-squared test on data from dihybrid crosses.
Theory of knowledge:
• The law of independent assortment was soon found to have exceptions when looking at linked genes. What is the difference between a law and a theory in science?