Chapter 7: Extending Mendelian Genetics

Chapter 7: Extending Mendelian Genetics

Chapter 11: Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance 11.2 Complex Patterns of Inheritance EQ: What is the difference between sex linked genes and genes found on autosomal chromosomes?

Chromosomes and Phenotype KEY CONCEPT: The chromosomes on which genes are located can affect the expression of traits. Gene expression is often related to whether a gene is located on an autosome or on a sex chromosome. All humans have 46 chromosomes

23 pairs 44 are autosomes (normal traits) Pairs 1-22 2 are sex chromosomes (X or Y)

XX = female XY = male Sex chromosomes determine an organisms sex Autosomes are the other chromosomes that DO NOT play a direct role in sex determination. Mendel Mendel studied autosomal

gene traits, like hair texture. Mendels rules of inheritance apply to autosomal genetic disorders. A heterozygote for a recessive disorder is a carrier. protein cystic fibrosis transmembrane Disorders conductance regulator

(CFTR). caused by This protein is required to regulate the dominant alleles are components of sweat, digestive fluids, uncommon. and mucus. Leads to thick, viscous secretions (dominant) Sex linked genes

Males and females can differ in sex-linked traits. Genes on sex chromosomes are called sexlinked genes. Y chromosome genes in mammals are responsible for male characteristics. X chromosome genes in mammals affect many traits. X chromosomes carry some genes that Y does not. Y is not large enough to carry

them Cause a difference of inheritance between males and females Sex linked genes cont. Male mammals have an XY genotype. All of a males sex- linked genes are

expressed. Males have no second copies of sexlinked genes. 7.1 Sex linked genes cont. 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.

XO =Orange Fur Xo = Black fur An in-depth look The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin (silencing of genes) X-inactivation prevents them from having twice as many X

chromosome gene products (RNA or proteins) as males, which only possess a single copy of the X chromosome. Abnormal amounts of gene products can be related to oncogenes which cause cancer. The choice of which X chromosome will be inactivated is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism. Sex linked genes cont.

Most of the sex linked genes are found only on the X chromosome Very few are found on the Y chromosome Cause difference in male & female inheritance Males will be affected or unaffected only Females will be affected, unaffected, or a carrier for these traits. Examples of X-linked traits

Found on the X chromosomes: redgreen color blindness, muscular dystrophy, baldness (male pattern baldness), hemophilia Found on the Y: SRY gene (maleness gene) Many genes that code for proteins are found on the Y gene Notice male chance is 50% and

female chance is 50% Female carriers can pass on a trait without showing that trait themselves Human Genetics and Pedigrees

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. Gene Expression

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. X Y

EQ: What is a Pedigree Chart and what is its significance? Pedigree Charts A pedigree is a chart for tracing genes in a family. What you need to

know in order to read a pedigree chart. Pedigree Charts Phenotypes are used to infer genotypes on a pedigree. Autosomal genes show different patterns on a pedigree than sex-linked genes.

Pedigree Charts cont... If the phenotype is more common in males, the gene is likely sex-linked. EQ: What role does complex patterns of inheritance play in organisms? 11.2 Complex Patterns of Inheritance

KEY CONCEPT: Phenotype is affected by many different factors. The rules are not always followed! There are some exceptions Phenotype can depend on interactions of alleles.

In incomplete dominance- heterozygous genes have a phenotype that is between the dominant and recessive traits Black + White = gray Also called intermediate inheritance Heterozygous phenotype is intermediate between the two homozygous phenotypes Homozygous parental phenotypes not seen in F 1 offspring Incomplete dominance

Incomplete dominance in humans Face shape Ear size Type of hair (curly, wavy. Straight) Nose size Mouth size Codominance

Codominant alleles will both be completely expressed (both dominant). Codominant alleles are neither dominant nor recessive. The ABO blood types result from codominant alleles.

Multiple Alleles Multiple Alleles More than 2 alleles may exist for a trait Blood Type A, B, O, & AB types IA, IB, i (3 alleles) Type AB DO NOT mask over

each other Variations Many genes may interact to produce one trait. Polygenic Inheritance when two or more genes affect a trait Examples: height, skin color, eye color Multiple genes contribute to the trait AaBbCc trait using three sets of genes

Some traits can be affected by dozens of genes Polygenic Inheritance Order of dominance: brown > green > blue. In humans, eye color is the result of polygenic inheritance. At the present, three gene pairs controlling human eye color are known. Two of the gene pairs occur on chromosome pair 15 and one occurs on chromosome pair 19. The

bey 2 gene, on chromosome 15, has a brown and a blue allele. A second gene, located on chromosome 19 (the gey gene) has a blue and a green allele. A third gene, bey 1, located on chromosome 15, is a central brown eye color gene. The brown allele is always dominant over the blue allele so even if a person is heterozygous (one brown and one blue allele) for the bey 2 gene on chromosome 15 the brown allele will be expressed. The gey gene also has two alleles, one green and one blue. The green allele is dominant to the blue allele on either chromosome but is recessive to the brown allele

on chromosome 15. This means that there is a dominance order among the two gene pairs. If a person has a brown allele on chromosome 15 and all other alleles are blue or green the person will have brown eyes. If there is a green allele on chromosome 19 and the rest of the alleles are blue, eye color will be green. Blue eyes will occur only if all four alleles are for blue eyes. Heterochromia iridum

In complete heterochromia, one iris is a different color from the other. In partial heterochromia or sectoral heterochromia, part of one iris is a different color from its remainder. Most cases of heterochromia are hereditary, caused by a disease or syndrome, or due to an injury. The pigment is produced in a specialized group of cells known as melanocytes. Alexandrias Genesis

Not Enough Scientific Evidence to support It is possible that there is a gene variation that can cause purple eyes, lighter skin and allows people to remain healthy so they live longer, but there is not necessarily enough evidence that this specific condition exists today. The traits of Alexandrias Genesis appear to be too widespread to be part of a single gene, which means they would have to stem from several unrelated genes. Some of the traits associated with this disorder are physiologically impossible. One could not fail to produce waste because toxins would build up in the body, causing death.

Fair skin that resists burning is also impossible because the only way to protect the skin is by the presence of melanin that makes the skin darker. Melanocytes/ Melanin Primarily responsible for skin color, also for eye color, and hair color. The difference in skin color between lightly and darkly pigmented individuals is due not to the number (quantity) of melanocytes in their skin, but to the melanocytes' level of activity Tyrosinase is required for melanocytes to produce melanin from the amino acid

tyrosine. Additional information Eumelanin and Pheomelanin Eumelanin: There are two types are recognized: black and brown. Black melanin is darker than brown. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown

eumelanin in the absence of other pigments causes yellow (blond) color hair. Pheomelanin: imparts a pink to red hue and, thus, is found in particularly large quantities in red hair. Pheomelanin is particularly concentrated in the lips and nipples. How they differ from each other is their chemical structure. Albinism

Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans. Albinos lack an enzyme called tyrosinase. Environmental Factors 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

(temperature dependent- sex determination) Occurs in embryonic stage. Males are produced at lower temperatures than females. Height is an example of a phenotype strongly affected by the environment.

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