Chromosomes and Heredity Chromosome theory of inheritance genes

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Chromosomes and Heredity

Chromosomes and Heredity

 • Chromosome theory of inheritance – genes are located on chromosomes, and chromosomes

• Chromosome theory of inheritance – genes are located on chromosomes, and chromosomes provide the basis for the segregation and independent assortment of genes.

Sex-linked Inheritance • Some traits that are passed from generation to generation depend on

Sex-linked Inheritance • Some traits that are passed from generation to generation depend on the sex of the parent carrying the trait. – This is because the genes for these traits are located on the sex chromosomes. • The transmission of genes that are located on one of the sex chromosomes, X or Y, is called sex-linked inheritance.

 • A gene that is located on the X chromosome only is called

• A gene that is located on the X chromosome only is called X-linked • A gene that is located on the Y chromosome only is called Y-linked • Most of the known sex traits are X-linked. – This may be because the Y chromosome is much smaller than the X chromosome. – Ex. Hemophilia, colour blindness.

Morgan’s Discovery • Thomas Morgan produced a white-eyed male fly by crossing 2 red-eyed

Morgan’s Discovery • Thomas Morgan produced a white-eyed male fly by crossing 2 red-eyed parent flies. • This result could be easily explained using the law of dominance in a monohybrid cross • When Morgan crossed the white-eyed male with a red-eyed female, he was not able to produce a female fruit fly with white eyes. • Hypothesized that the gene coding for eye colour in fruit flies was located on the X chromosome

 • The white-eye trait must be recessive. Therefore, the only way to obtain

• The white-eye trait must be recessive. Therefore, the only way to obtain a female white eyed fruit fly was if both male and female parent donated an allele that coded for white eyes. • The male donates the Y chromosome that does not carry the colour gene to all male offspring. The female donates the X chromosome to both the male and female offspring. • The source of the other X chromosome comes from the male, and the X chromosomes contain the allele that codes for eye colour.

Sex-Linked Traits and Punnett Squares • When creating a Punnett square to determine the

Sex-Linked Traits and Punnett Squares • When creating a Punnett square to determine the outcome of a cross involving a sex-linked trait, assume that the trait is located on the X chromosome, unless it is stated that it is located on the Y chromosome. • The gametes containing the X and Y chromosomes are placed on the outside of the Punnett square (same as Mendelian genetics).

 • The only differences are that the alleles are written on the X

• The only differences are that the alleles are written on the X chromosome if the traits are X-linked and no allele is written on the Y chromosome. • When the genotypic and phenotypic ratios are written, the sex is always included in the ratio.

 • Ex. Colour blindness is a recessive allele carried on the X chromosome.

• Ex. Colour blindness is a recessive allele carried on the X chromosome. If a woman who sees colour and is heterozygous for that trait mates with a man who also has normal sight, what is the probability that their children will be affected or carriers?

 • Ex. Hemophilia is a recessive disorder carried on the X chromosome. If

• Ex. Hemophilia is a recessive disorder carried on the X chromosome. If a woman who has hemophilia mates with a man who does not, what is the probability that their children will be affected or carriers?

 • Ex. Baldness is a recessive sex-linked trait. If a man who is

• Ex. Baldness is a recessive sex-linked trait. If a man who is bald mates with a woman who has hair, but is a carrier for that trait, what are the odds that their children will be bald or carriers for the baldness trait?

Chromosomes and Gene Expression • Males and females produce the same amount of proteins

Chromosomes and Gene Expression • Males and females produce the same amount of proteins coded by genes on the X chromosome. However, females have 2 copies of the X chromosome in every cell, and males only have one copy of the X chromosome in every cell. • One of the X chromosomes in every female cell is inactivated. Whichever one becomes inactive is random, and therefore different X chromosomes are active in different cells. • The inactive X chromosome is called a Barr body. • Ex. Calico coat colour in cats (random distribution of orange and black patches).

 • Can only occur in female cats • Gene that codes for orange

• Can only occur in female cats • Gene that codes for orange and black colour is located on the X chromosome • A tortoiseshell cat is heterozygous for the allele that codes for this coat colour • One copy of the X chromosome has an allele that codes for orange pigment, and the copy of the X chromosome has an allele that codes for black pigment. • The random inactivation of an X chromosome in every cell of a tortoiseshell cat results in a patchwork of orange and black colour.

Polygenic Inheritance • Many traits are controlled by more than one gene – Polygenic

Polygenic Inheritance • Many traits are controlled by more than one gene – Polygenic Inheritance • The protein products produced by many genes working together results in a range of variation – continuous variation. • Continuous variation – variation among individuals in a population in which there a gradient of phenotypes for one trait.

 • Ex. Ear length in corn. • Longest ear length obtained when both

• Ex. Ear length in corn. • Longest ear length obtained when both genes controlling corn ear length are homozygous dominant for both alleles. • The shortest ear length occurs when the genotype is homozygous recessive for both alleles. • Longest and shortest are least common because each can only be obtained by one combination of alleles. • In humans, height and skin colour are thought to be polygenic traits.

Modifier Genes • Modifier genes – genes that work along with other genes to

Modifier Genes • Modifier genes – genes that work along with other genes to control the expression of a trait. • Ex. The expression of eye colour in humans may have modifier genes. • Brown eye colour is a result of melanin (pigment) that is coded for by the dominant allele for eye colour. • Blue eye colour is the result of the absence of the pigment melanin because blue eyes are a recessive trait. • Various modifier genes help to produce other eye colours.