Two Copies of Each Autosomal Gene Affect Phenotype

Chapter 7 Notes

7-1 • Two copies of each autosomal gene affect phenotype. • Mendel studied autosomal gene traits, like hair texture. • Mendel’s rules of inheritance apply to autosomal genetic disorders.

• Males and females can differ in sex-linked traits. • Genes on sex chromosomes are called sex-linked genes. – Y chromosome genes in mammals are responsible for male characteristics. – X chromosome genes in mammals affect many traits. The X chromosome is much larger than the Y, and has many more genes.

 A recessive disorder that is carried on the X chromosome is said to be sex-linked. Example: colorblindness This condition is much more common in males because you only have to have the recessive allele on the X chromosome for the male to have colorblindness. (males only have the 1 X chromosome, unlike females whose second X could contain the dominant allele, so she will only be a carrier).

Male – XcY (colorblind male) Female – XCXc (carrier – does not have the trait, but can pass allele on to offspring)

Ex: XcY X XCXc

Other examples of sex-linked recessive disorders include: hemophilia and muscular dystrophy.

7-2 • Phenotype can depend on interactions of alleles. • In incomplete dominance, neither allele is completely dominant nor completely recessive. – Heterozygous phenotype is intermediate between the two homozygous phenotypes

– Homozygous parental phenotypes not seen in F1 offspring

** Use all capital letters to show different from the traditional cross R – red W – white

Ex. RW X RW

Genotypic ratio? 1 RR : 2 Rr : 1 rr Phenotypic ratio? 1 red : 2 Pink : 1 white Codominance – both alleles are dominant. Codominant alleles will both be completely expressed. They both contribute to the phenotype of the organism. So both are showing. Ex. In cows or horses, both red hair and white hair may be dominant. Both show up in the phenotype, so the animal appears roan. • Many genes may interact to produce one trait. • Polygenic traits are produced by two or more genes.

Multiple alleles – occur when there are more than 2 different alleles for a trait Ex. blood types are A, B, AB, and O ** Use a capital I in these problems to show that they are multiple alleles

A = IAIA or IAi B = IBIB or IBi AB = IAIB O = ii

Ex. Hetero. A X O

• The environment interacts with genotype.

7-3 • Gene linkage was explained through fruit flies. • Morgan found that linked traits are on the same chromosome. • Chromosomes, not genes, assort independently during meiosis. • Linked genes are not inherited together every time. Ex. reddish-orange eyes are almost always going to have miniature wings • each chromosome is actually a group of linked genes • Mendel’s principle of independent assortment still holds true • ** It is the chromosomes that assort independently, NOT individual genes

• Linkage maps estimate distances between genes. • The closer together two genes are, the more likely they will be inherited together. • Cross-over frequencies are related to distances between genes. • Linkage maps show the relative locations of genes. • If 2 genes are on the same chromosome and rarely assort independently, the genes are probably located close to each other.

7-4 • Human genetics follows the patterns seen in other organisms. • The basic principles of genetics are the same in all sexually reproducing organisms. – Inheritance of many human traits is complex. – Single-gene traits are important in understanding human genetics.

• Several methods help map human chromosomes. • A karyotype is a picture of all chromosomes in a cell grouped from largest to smallest in homologous pairs. A karyotype shows: homologous chromosomes, sex chromosomes, and autosomes. There are 23 pair in humans. Sex chromosomes determine an individual’s sex. The 23rd pair are the sex chromosomes. Females – XX Males – XY All chromosomes that are not sex chromosomes (pair 1 – 22, are autosomes). The female egg carries the X. Male sperm carry an X or a Y. So, males determine the sex of the child.

Chromosomal Disorders: Nondisjunction – homologous chromosomes fail to separate during Prophase I of meiosis (most common error in meiosis). If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result. Example: Down’s syndrome – Trisomy 21 - 3 copies of chromosome pair #21 Example of sex chromosome disorders: Turner’s syndrome (XO) and Klinefelter’s Syndrome (XXY).

• A pedigree is a chart for tracing genes in a family. • Phenotypes are used to infer genotypes on a pedigree. • Autosomal genes show different patterns on a pedigree than sex-linked genes.

A pedigree chart shows the relationships within a family. Circle = female square = male horizontal line = marriage vertical line= children completely shaded = expresses trait partially shaded = carrier only not shaded = trait not expressed/not a carrier

• Females can carry sex-linked genetic disorders. • Males (XY) express all of their sex linked genes. • Expression of the disorder depends on which parent carries the allele and the sex of the child. Carrier – a person who does not have a trait, but does carry the allele for the trait, and can pass it on to his/her offspring.  Remember that the phenotype of an organism is only partially determined by its genotype. Many traits are strongly influenced by environmental factors.  Examples: nutrition and exercise