EXTENDING MENDELIAN GENETICS Biology 1 Chapter 7 7

EXTENDING MENDELIAN GENETICS Biology 1 Chapter 7

7. 1 CHROMOSOMES AND PHENOTYPE • The chromosome that a gene is located on affects the phenotype of the trait. • Gene expression is often related to whether it was located on an autosome or a sex chromosome. • Sexually producing organisms have 2 of each chromosome forming a pair. • Each of the chromosomes in the pair came from each parent.

• While both of the chromosomes in the pair have the same genes, they may have different alleles for the gene. • Most of the traits in sexually producing organisms are found on autosomal genes. • Many genetic disorders are also caused by autosomal genes.

• Remember that all of the chromosomes except for pair 23 are autosomes. • You have a homologous pair of each autosome. • Same size, shape, and genes • Any gene that is on one autosome will also be on matching autosome. • That gives you 2 copies of each gene and either one can influence phenotype.

• Some genetic disorders are caused by recessive alleles on autosomes. • For the person to have this type of disorder, the recessive allele must be present on both autosomes. • These disorders usually appear in the offspring of parents who were both heterozygous. • The parents do not have the condition because the dominant normal allele masked the disease causing recessive allele.

• A person who is heterozygous does not have the disease but is a carrier. • A carrier does not show the disease symptoms but can pass on the allele. • A person that has a condition that is homozygous recessive will pass on the allele to their offspring. • Cystic fibrosis is an example of a recessive disorder

• Dominant genetic disorders are much less common. • If a disease is caused by a dominant allele there is a 50% chance that a child will have it even if only one parent has the allele. • If both parents are heterozygous, then there is a 75% chance that any of their children will have the disease. • Huntington’s disease is an example of a dominant disorder.

• Genes that are located on sex chromosomes are called sex-linked genes. • Recall that the sex chromosomes in humans are X and Y. • Genes on the Y chromosome are responsible for the development of the male offspring. • The X chromosome has a lot of influence on traits.

• The Y chromosome is much smaller and has fewer genes than the X chromosome. • Because the X and Y chromosomes have different genes, sex linked genes have a different pattern of expression.

• Because males only have one copy of each type of sex chromosome, they will express all of the alleles on each chromosome. • There is no second copy on the matching chromosome that can mask the effects of the allele. • This means that even if the allele is recessive, they will still be expressed.

• In each cell of the female, one of the two X chromosomes is randomly turned off by a process called X chromosome inactivation. • Because of this, females are a patchwork of 2 types of cells: • One with an active X chromosome from the mother. • One with an active X chromosome from the father.

• Because females have two X chromosomes, the dominant and recessive alleles factor into whether or not the female will express a trait. • If a trait is recessive, then both X chromosomes have to have the recessive form of the allele in order for the trait to be expressed.

7. 2 COMPLEX PATTERNS OF INHERITANCE • Remember that dominance when it comes to traits does not mean that one allele defeats the other. • It means that the dominant allele codes for a certain protein and a recessive allele codes for a variation of the protein that has little or no effect.

• In many cases, a phenotype comes from more than one gene. • Many genes have more than just 2 alleles.

• Sometimes alleles show incomplete dominance. • Incomplete dominance occurs when a heterozygous phenotype is somewhere in between the two homozygous phenotypes.

• If two organisms that display incomplete dominance are crossed, the generation produced can be predicted: • 25% will have one of the homozygous traits • 25% will have the other homozygous trait • 50% will have the incomplete dominant form of the trait

F 1 Generation

• Codominance occurs when both alleles of a gene are completely expressed. • Human blood types are examples of codominance. • Blood type is based on proteins called antigens. • There are 3 possible alleles for blood type: • IA , IB, and i • IA and IB are codominant. • i is recessive to IA and IB.

• IAIA and IAi will produce type A blood. • IBIB and IBi will produce type B blood. • IAIB will produce type AB blood. • ii will produce type O blood.

• Traits produced by 2 or more genes are called polygenic traits. • Skin color – 4 genes • Eye color – 3 genes • Epistatic genes interfere with the expression of other genes. • Albinism

• In some organisms, the environment can play a role in determining the sex of the offspring (sea turtles). • Studies of identical twins have shown that the environment during early development can have long lasting effects on height and size. • Nutrients obtained in the uterus • Nutrition and health care differences

7. 3 GENE LINKAGE AND MAPPING • Some traits appear to be inherited together. These are called linked traits. • Linked genes are on the same chromosome. • Chromosomes assort independently during meiosis. • Linked genes are not inherited together every time which means that homologous chromosomes must exchange genes (crossing over) during meiosis.

• The probability that two genes will be inherited together is related to the distance between them. • The closer two genes are, the more likely that they will be inherited together. • The farther apart they are, the more likely they will be separated. • The closer two genes are, the more likely they were to stay together during crossing over.

• Linkage maps are maps of relative locations (loci) of genes on a chromosome. • When traits cross-over, they do not appear together in the offspring. • The lower the percentage of cross-overs between 2 genes, the closer together these genes are on the chromosome.

7. 4 HUMAN GENETICS AND PEDIGREES • A carrier of an autosomal disorder does not show the disease but can pass on the disease causing allele. • Both males and females can be carriers of autosomal disorders. • Only females can be carriers of sex-linked disorders. • Males will have or not have the disorder. • Several genetic disorders are caused by genes on the X chromosome.

• Females can be just carriers because they contain two X chromosomes. • Males cannot be carriers of sex linked disorders because they only have one copy of the X chromosome. • The likelihood of inheriting a sex linked disorder depends on both the sex of the child and which parent carries the allele.

• A pedigree is a chart that can help trace the phenotypes and genotypes in a family. • They are used to determine whether people carry recessive alleles. • If enough family phenotypes are known genotypes can be inferred.

• Rules for pedigrees: • Boxes represent males and circles represent females. • A shaded shape means the person shows the trait. • A white shape means the person does not show the trait. • A shape that is half shaded means the person is a carrier.

• If there approximately the same number of males and females have the trait, then it is most likely found on an autosome. • If the trait is more common in males, then the gene is most likely found on the X chromosome.

• When tracing a genotype where the trait is controlled by an autosomal gene you can tell that the person is homozygous recessive if they do not show the trait. • Homozygous dominant and heterozygous would show the trait. • With autosomal traits there are no carriers. You either have it or you don’t.

• When tracing a sex linked trait you have to look at the sex chromosomes and dominant and recessive alleles. • More males than females show sex-linked traits. • Females can be carriers of sex linked traits. • By using a process of elimination you can figure out the possible genotypes for a given phenotype.

Pedigree

• A karyotype is a picture of all the chromosomes in a cell. • In order to study the chromosomes, chemicals are used to stain them which produces a pattern of bands. • Karyotypes can be used to locate genes on chromosomes or estimate the distance between genes. • Karyotypes can show changes in chromosomes. • Too many chromosomes or missing parts of chromosomes.

Karyotype

• The Human Genome Project is an international effort to map, sequence, and identify all of the genes in the human genome.