Mendelian Inheritance of Human Traits Chapter 12.1

Making a Pedigree Have you ever seen a family tree? What does a family tree show? It shows the relationships among family members. It shows how grandparents, parents, children, aunts, uncles, cousins, and siblings are related. Geneticists are scientists who study how inherited or genetic traits pass from one generation to the next. Sometimes they use what is called a pedigree to study a family’s genetic puzzle. A pedigree is a visual diagram of genetic inheritance used by geneticists to map genetic traits. A pedigree uses a set of symbols to identify males and females in a certain family that carry the genetic trait being studied.

circles represent females squares represent males. Shaded circles and squares represent those who have the trait that is being studied. Unshaded circles and squares represent individuals who do not have the trait. half-shaded circle or square represents a carrier—someone who has a recessive allele for a specific trait. A circle and a square connected by a horizontal line represents parents. A vertical line connects parents with their offspring.

Most genetic disorders are caused by recessive alleles. cystic fibrosis, Tay-Sachs disease, phenylketonuria, PKU.

Simple Dominant Heredity A cleft chin, a widow’s peak hairline, freely hanging earlobes, almond-shaped eyes, and thick lips are examples of dominant traits. Only one allele needs to be present for these traits to show up.

Huntington’s disease is caused by a rare dominant allele. There is no effective treatment for Huntington’s disease, which causes a breakdown in certain parts of the brain. Because Huntington’s disease doesn’t occur until a person is between the ages of 30 and 50, many people have already had children before they develop the disease. A pedigree could help people with Huntington’s disease in their family better understand their own risks for the disease and for passing it on to future generations.

Incomplete Dominance When the phenotype of the heterozygous offspring is a mix of the two parents. For example, when a homozygous red flower is crossed with a homozygous white flower , the offspring’s color will be a mix of the two. It will be pink. Ratio is 1:2:1.

Codominant Inheritance In codominant inheritance, both alleles show up equally. Codominant alleles cause the phenotypes of both homozygote parents to be expressed equally in the heterozygote offspring. For example, when a certain variety of black chicken is crossed with a white chicken, all of the offspring are checkered(Some are black and some are white)

Non-Mendelian traits

How do multiple alleles work? In some populations, traits can be controlled by multiple alleles. This means there are more than two alleles for a genetic trait. For example, many combinations of pigeon feather colors exist. The allele for ash-red colored feathers is dominant. The allele for wild-type blue feathers is recessive to the allele for ash-red feathers. The allele for chocolate-brown feathers is recessive to both the ash-red and the wild-type blue alleles. Sometimes there are as many as 100 alleles for a single trait!

What determines the sex of an organism? Humans have 23 pairs of chromosomes. Twenty-two of these pairs of homologous chromosomes are called autosomes. Homologous autosomes look alike. The 23rd pair of chromosomes is called the sex chromosomes. female XX male XY.

What are sex-linked traits? Sex-linked traits are the traits controlled by genes located on sex chromosomes. Eye color in fruit flies is an example of an X-linked trait. This means eye color in fruit flies is determined by a gene on the X chromosome. X-linked traits are passed to both males and females. Y-linked traits are passed only to male offspring because the genes for these traits are on the Y chromosome.

What is polygenic inheritance? Some traits, such as skin color and height in humans, vary over a wide range. This is because the traits are controlled by many genes rather than by just one gene. Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes. The genes may be on the same or different chromosomes.

Environmental Influences It is important to know that the genetic makeup of an organism at fertilization determines only the organism’s potential to develop and function. Many factors can influence how the gene is expressed, or whether the gene is expressed at all. There are internal and external influences.

How does external environment affect organisms? Nutrition, light, chemicals, infectious agents such as bacteria, fungi, parasites, and viruses, and other factors can all influence the ways genes are expressed. The arctic fox, for example, has gray-brown fur in warm temperatures. When temperatures fall, the fur becomes white. In this case, temperature is the external factor that affects the phenotype of fur color.

How does internal environment affect organisms? The internal environments of males and females are different because of hormones and structural differences. Horn size in mountain sheep is expressed differently in males and females. In males, the horns are much heavier and more coiled than the horns of females. The age of an organism can also affect the way genes function. The internal environment of an organism changes with age, but it is not clearly understood how these changes affect the function of genes.

Complex Inheritance of Human Traits Chapter 12.3

Do now: What is Codominance? What is sickle-cell anemia?

Sickle-cell anemia is a major health problem in the United States and in Africa. It is most common in African Americans and in white Americans whose families came from countries around the Mediterranean Sea. In a person who is homozygous for the sickle-cell allele, abnormal sickle-shaped red blood cells are produced. Normal red blood cells are disc shaped. Sickle-shaped cells occur in the body’s narrow capillaries. They slow blood flow, block small vessels, and cause tissue damage and pain. Because of the short life span of the sickle cells, individuals with this disease have several related disorders. Individuals who are heterozygous for the sickle-cell allele produce both normal and sickle-shaped red blood cells. This is an example of codominance. These individuals produce enough normal red blood cells that they do not have the serious health problems of those individuals who are homozygous for the allele. Individuals who are heterozygous for the allele can lead relatively normal lives.

The ABO blood group is a good example of a single gene that has multiple alleles in humans. Human blood types are determined by the presence or absence of certain molecules on the surface of red blood cells.

Why is blood typing important? Blood typing determines the ABO blood group to which an individual belongs. If the wrong blood type is given, the red blood cells could clump together and cause death. Blood typing can also be helpful in cases of disputed parentage. For example, if a child has type AB blood and his or her mother has type A blood, a man with type O blood could not be the father.

What is Sex-Linked Traits ? Ex: Red-green Color Blindness Hemophilia

red-green color blindness = unable to tell the difference between these two colors. caused by the inheritance of a recessive allele on the X chromosome. traffic lights

What is hemophilia? If you have ever cut yourself, you may have noticed that cuts usually stop bleeding quickly. The cut stops bleeding because the blood clots. Hemophilia is an X-linked disorder that keeps blood from clotting quickly. It also means that a person could have internal bleeding from a bruise. Hemophilia can be treated with blood transfusions and injections of the blood-clotting enzyme that is absent in people with hemophilia. (very expensive)

Polygenic Inheritance in Humans In the early 1900s, scientists studied data collected on skin color. They found that when light-skinned people mate with dark-skinned people, their children have intermediate skin colors. When the children have offspring, the skin colors in the offspring range from the light to the dark of the grandparents. Most of the offspring, however, have an intermediate skin color. This variation indicates that three to four genes are involved in determining skin color.

Changes in Chromosome Numbers Sometimes there are an abnormal number of cells in a set of chromosomes. To identify an abnormal number of chromosomes, a sample of cells is taken from an individual or a fetus (a developing mammal from nine weeks old to birth). The chromosomes are photographed and arranged in pairs by a computer. The pairs are arranged by length and location of the centromere. The chart showing the pairs is called a karyotype. It is very useful in identifying unusual chromosome numbers in cells

What is Down syndrome? Most human abnormal chromosome numbers result in embryo death, usually even before a woman knows she is pregnant. Down syndrome is one example of an abnormal number of chromosomes in humans. Individuals with Down syndrome can survive to adulthood, unlike most others born with an abnormal number of chromosomes. Individuals with Down syndrome have at least some degree of mental retardation. The incidence of Down syndrome births is higher in older mothers, especially those over 40 years old.

What happens with abnormal numbers of sex chromosomes? Sometimes an X chromosome will be missing (designated XO). There may also be an extra chromosome, such as in XXX or XXY. There may even be an extra Y chromosome (XYY). Any individual with a Y chromosome is a male. Any individual without a Y chromosome is a female. What happens in these cases? Most of these individuals lead normal lives but cannot have children. Some have varying levels of mental retardation.