KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits.

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. A heterozygote for a recessive disorder is a carrier. Disorders caused by dominant alleles are uncommon. (dominant)

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.

Male mammals have an XY genotype. The X chromosome is much larger than the Y, and has more genes. All of a male’s sex-linked genes are expressed. Males have no second copies of sex-linked genes.

Female mammals have an XX genotype. Expression of sex-linked genes is similar to autosomal genes in females. X chromosome inactivation randomly “turns off” one X chromosome. (Barr bodies)

Females can carry sex-linked genetic disorders. (males CANNOT) 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. Females must get the recessive gene from both parents to have a sex-linked disorder. X Y

Cross these Parents: Male – Xcy (colorblind) Xc Female – XCXc A recessive disorder that is carried on the X chromosome is said to be sex-linked. Ex. Red-green colorblindness, hemophilia, muscular dystrophy The condition is much more common in males because the male only has to have the recessive allele on the X chromosome (he gets from his mom) Unlike females, the male does not have a second X to be dominant over it. XC Xc Cross these Parents: Male – Xcy (colorblind) Female – XCXc (Carrier) XCXc XcXc XCy Xcy Xc y

Other examples of sex-linked disorders: hemophilia and muscular distrophy

KEY CONCEPT Phenotype is affected by many different factors.

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 (blends) Homozygous parental phenotypes not seen in F1 offspring

Incomplete dominance example: Use all capital letters to show different from traditional cross Ex: R – red W – white RW - pink Cross these Parents: Male – pink Female – pink Genotypic ratio: 1RR:2RW:1WW Phenotypic ratio: 1red:2pink:1white R W RR RW WW R W

Codominant - 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, bot red hair and white hair may be dominant. Both show so the animal appears roan. The AB blood type in humans result from codominant alleles.

Blood types in humans: A, B, AB, O Multiple alleles: occur when there are more than 2 different alleles for a trait. Blood types in humans: A, B, AB, O Use an “I” to show multiple alleles. Ex. Hetero A X O i i IAi ii IA i

Many genes may interact to produce one trait. Polygenic traits are produced by two or more genes. Order of dominance: brown > green > blue.

The environment interacts with genotype. Phenotype is a combination of genotype and environment. The sex of sea turtles depends on both genes and the environment Height is an example of a phenotype strongly affected by the environment.

KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

Gene linkage was explained through fruit flies. Morgan – used fruit flies and found that linked traits are on the same chromosome. Chromosomes, not genes, assort independently during meiosis. Linked genes are not inherited together every time. Wild type Mutant

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.

KEY CONCEPT A combination of methods is used to study human genetics.

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.

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.

If the phenotype is more common in males, the gene is likely sex-linked.

Tracing Autosomal Genes Tracing Sex-linked Genes Equal numbers of males and females have it More males than females will have it; females can be carriers People with recessive phenotype must be homozygous recessive Females with recessive phenotype have two recessive alleles; males only have one People with dominant phenotype can be homozygous dominant or heterozygous Heterozygous phenotypes don’t show the recessive phenotype – are carriers Two heterozygotes can have offspring of either phenotype Female carriers can pass on recessive allele to either male or female offspring Or genotype Males with recessive can pass on recessive allele only to females

Several methods help map human chromosomes. A karyotype is a picture of all chromosomes in a cell. Karyotypes show: homologous chromosomes, sex chromosomes, and autosomes. There are 23 pair of homologous chromosomes in humans. Sex chromosomes determine an individual’s sex. The 23rd pair are the sex chromosomes. Females – XX Males – XY Pair 1 – 22 are autosomes. X Y

Karyotypes can show abnormalities in the chromosomes. deletion of part of a chromosome or loss of a chromosome large changes in chromosomes extra chromosomes or duplication of part of a chromosome

Nondisjunction – homologous chromosomes fail to separate during Prophase I. Abnormal numbers of chromosomes find their way into gametes, and a disorder of chromosome numbers may result. Examples: Down’s Syndrome – Trisomy 21 Turner Syndrome – missing a sex chromosome (XO) Klinefelter’s Syndrome – extra sex chromosome (XXY) Others: XXX, XYY

The female egg carries the X. Male sperm carry an X or a Y The female egg carries the X. Male sperm carry an X or a Y. So males determine the sex of the child.