Patterns+of+Heredity+and+Human+Genetics

=**Mendelian Inheritance of Human Traits**= Pedigrees

Pedigrees show the genetic inheritance of a certain trait. Pedigrees are formatted almost exactly like family trees, but girls are shown as circles and boys are shown as squares. If they have the trait which is being displayed, the shapes are shaded in a certain color. Half-shaded shapes show “carriers”, which are individuals that are heterozygous for the trait being examined. Horizontal lines drawn between individuals indicate that they’re parents, and vertical lines connect the parents to their offspring. The generations are labeled with Roman numerals, and every individual within a generation is given a number. Geneticists use the information to analyze the genetics within a specific family.



__Simple__ Recessive __Heredity__ Phenylketonuria Phenylketonuria (PKU) is an autosomal recessive disorder that results from the absence of an enzyme that converts one amino acid, phenylalanine, into another, tyrosine. Since phenylalanine is not broken down, it and its biproducts build up in the body, causing severe damage to the central nervous system. This allele is most common in the United States, in people descendants from Norway, Sweden or Ireland. A newborn that is homozygous recessive for PKU at first appears healthy, however once the child begins drinking milk, which is rich in phenylalanine, the amino acid accumulates and mental retardation often occurs. Today PKU tests are taken shortly after a child is born. If these tests come out positive the child is put on a diet containing foods with very little phenylalanine until the brain is able to fully develop. This prevents the retardation of the child. There is another problem that occurs in females who are homozygous recessive for PKU. If a woman with the trait becomes pregnant the high levels of phenylalanine can damage the fetus. This occurs even if the child is phenotypically normal.

Tay-Sachs
Tay-Sachs is a recessive disorder of the central nervous system. The disorder comes about when a recessive allele is the outcome from a missing enzyme. This missing enzyme would normally break down a liquid which is made and then stored in the tissues of the central nervous system. Because this is unable to happen, it accumulates in the cells. The allele for Tay-Sachs is most common in the U.S. among Ashkenazic Jews, whose family originated in Eastern Europe.

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Simple Dominant Heredity: Simple Dominant Heredity- Requires only one Dominant allele from only one parent Simple Dominant Traits- A dominant trait comes from a dominant allele inherited from one parent. Dominant traits include things like a widow's peak, a hitchhiker's thumb, almond-shaped eyes, and thick lips. A recessive trait is heterozygous, whereas a dominant trait is homozygous dominant. A heterozygous recessive trait could be shown as (ff), a heterozygous dominant trait could be shown as (Ff) and a homozygous dominant trait could be shown as (FF).

Huntington’s disease: Huntington's disease is a lethal genetic disorder caused by the dominant allele. It results in a breakdown of areas of the brain. No effective treatment exists. Normally, a dominant allele with extreme effects would result in death before the allele could be passed on to the next generation. But, because the onset of Huntington's disease usually occurs between the ages of 30 and 50, a person may have already had children before knowing he/she is affected. A genetic test has been developed that detects the presence of this allele. Although this test allows individuals with the allele to decide whether they want to have children and have the risk of passing the trait on to future generations, it also means that they know they will develop the disease. For this reason, some people may choose not to be tested.

[|pet.radiology.uiowa.edu/ webpage/Research/Case...]

= When Heredity Follows Different Rules = Complex Patterns of Inheritance

Sex Determination In humans, the diploid number of chromosomes is 46 (23 pairs). There are 22 pairs of homologous chromosomes called __autosomes__. Homologous autosomes look alike. The 23rd pair of chromosomes differs in males and females. These two chromosomes (they determine the sex) are called __sex chromosomes__. They are indicated with the letters X and Y. If you are female, 23rd pair of chromosomes is shown as XX (homologous). If you are male, they are shown as XY because they look different. Males usually have one X and one Y chromosome. Females have two X chromosomes. Males produce two kinds of gametes, X and Y and females produce only X gametes. The male gamete determines the sex of the offspring. After fertilization, a 1:1 ratio of males to females is expected. Because fertilization is governed by the laws of probability, the ratio isn’t usually 1:1 exactly in a small population. Autosomes: pairs of matching homologous chromosomes in somatic cells Sex Chromosomes: in humans, the 23rd pair of chromosomes determine the sex of an individual and carry sex-linked characteristics

Sex Linked Inheritance
Fruit flies inherit sex chromosomes the same way that humans do. Traits controlled by genes positioned on sex chromosomes are called sex-linked traits. In 1910 Thomas Morgan discovered traits linked to sex chromosomes. He noticed one day that a male fly had white eyes, instead of typical red eyes. He bred this white eyed fly with a homozygous red eyed female. All of the offspring had red eyes, showing white eyes are recessive. He allowed the offspring to mate amongst themselves, and according to Mendelian inheritance, the trait would reappear. There was a three to one ratio of white eyes, and only males had white eyes. He then bred a heterozygous female with a white eyed fly, and half of all females and males had white eyes. The allele for eye color is carried on the X and Y chromosome, making it a sex-linked trait.

Incomplete Dominance Incomplete dominance follows mixing patterns. Features such as pigment are often incompletely dominant, featuring crossing of the patterns. Instead of either being Red or Yellow, you obtain an orange colour. Incomplete Dominance often occur when both alleles are dominant, and both code for corresponding enzymes. This causes the effects of the enzymes to cross. The different alleles while both dominant have an apostrophe to determine their difference, expressed as an allele such as R and R’.

Two different uppercase letters, like A and Z, are used to represent the alleles in codominant inheritance. Bases on Mendel’s pea experiments, you would expect a heterozygous chicken, AZ, would be white. //Codomaninant alleles// cause the phenotypes of both homozygotes to be produced in heterozygous individuals.
 * Codominance: Expression of Both Alleles, ** Example- Chickens, black-feathered and white-feathered. A: White Z: Black

Although each trait has two alleles in a trait, and only one controls it, very often it happens that more than two alleles can control a trait. This happens because a new allele can be created at anytime because of a mutation. Traits that are controlled by two alleles have multiple alleles, there can be anywhere from three alleles to up to one hundred alleles.

Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes. Such traits include skin color, a person’s height, and other traits that are controlled by many genes. These genes may or may not be on the same chromosome, and each gene may have two or more alleles. Each allele has an equal amount of control over the trait being expressed, and this results in varied outcomes from the mixture of the alleles. The recessive alleles serve as the base on which the dominant alleles build, depending on how many dominant alleles there are, to create the end product with varied results.

Environmental Influences

So you think you know all about how genes are shown in a person? The genetic makeup of an organism at fertilization can only determine how the organism develops and functions. While an organism develops, lots of things can influence how genes are expressed. These “environmental influences” consist of an organism’s internal and external environments.

 __**Influence of External Environment**__ There are different external influences of the environment on heredity. Some examples can be temperature, nutrition, chemicals, light, and also infectious agents. They all influence genes in a variety of ways. In arctic foxes and snowshoe hares, temperature plays a role in coat color. Leaves have different shapes, thicknesses, and sizes depending on the amount of light they receive.

 **__Influence of Internal Environment__**  The internal environment of male and female organisms are different because of hormone and structural differences. For an example, horn size in mountain sheep is shown differently in males and females. We can see this in humans in Male-pattern baldness, and in peacocks, in feather color. These traits are all different based upon which sex an organism is. They are controlled primarily on hormones and genes. An organism's age can also affect gene functions. The nature of such pattern is not well understood, but it is known that the internaa envitonment of an organism changes with age. Understanding how genes interact with eachother and

The inheritance pattern is still not complete even when you understand dominant and recessive genes and you have solved the puzzles of the other patterns of heredity. The genetic makeup of an organism at fertilization only determines the potential of the organism to develop and function. As the organism develops many factors can influence how the gene is expressed or even if the gene is expressed at all. Two such influences are the organism’s external and internal environments.

Good Resource Websites Explaining Environmental Influence and Genetic Variation
1. [] 2. []

= Complex Inheritance of Human Traits = Codominance in Humans



Blood Typing

Blood typing is important because if you get the wrong type, you will die. Blood type is an inherited trait. There are four possible phenotypes. Ia and Ib are dominant, and ii is recessive. Dominant determines surface molecules. The importance of blood typing is so if a person is hurt, they can receive a blood transfusion without getting the wrong kind of blood, which is fatal.

Sex-Linked Traits in Humans

Genes are carried on through sex chromosomes. Males have XY chromosomes, whereas females have XX chromosomes. Males pass their X chromosomes to their daughters and their Y chromosomes to their son. The females pass their X chromosome to both their daughter and their son.

 **Red-Green color blindness**  This happens because you inherit a recessive allele at two gene sites of the X chromosome. Red-green color blindness is when you can no differentiate between the colors red and green.   **Hemophilia: An X linked disorder**  Hemophilia A is a problem with blood clotting. Without it, it would take a long time to stop bleeding when you get a cut or if you have a bruise, which is internal bleeding that won’t stop. Males inherit from the allele for Hemophilia on the X chromosome from their mothers. One recessive allele will cause this disorder. While females need two recessive alleles to get hemophilia. It can be treated with blood transfusions and injections of Factor VIII, the blood clotting enzyme that is absent in those with Hemophilia A.

Super References for Sex-linked Traits in Humans

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Polygenic Inheritance in Humans

Most of the traits you have received from your parents were inherited through simple Mendelian patterns or multiple alleles. The other traits are determined by polygenic inheritance. The traits that are determined by polygenic inheritance usually represent a range of variation and it is measurable. Examples of this trait are the variations of eye color and skin tone. Skin color was experimented with to prove it was a polygenic trait. The theory was that if people with darker skin would reproduce with people with lighter skin, the offspring would have skin with the color in between. When the second generation reproduces, then the skin color of the third generation will range from as light as the first generation to as dark as the first generation, but the majority is still of the medium skin color. The variation in skin color tells us that three to four genes were involved in the determination of the phenotype.
 * Polygenic Inheritance in Humans **

Here are some websites that could help: http://en.mimi.hu/biology/polygenic_inheritance.html http://waynesword.palomar.edu/lmexer5.htm http://www.wisegeek.com/what-is-a-polygenic-inheritance.htm http://staff.jccc.net/pdecell/evolution/polygen.html http://library.thinkquest.org/28751/review/heredity/11.html

Changes in Chromosomes Numbers So far, we know that traits are caused by one or a lot of genes on chromosomes. However, what would happen if the entire chromosome or part of it decided to walk off and disappear? Or, what would happen if a cell had an extra chromosome? Strange numbers of chromosomes in offspring is usually (though not always) caused by accidents in meiosis.

Abnormal Numbers of Autosomes A method of identifying abnormal number of chromosomes is to take a sample of cells obtained from a fetus. They take pictures of metaphase chromosomes and then they are arranged in pairs in a computer by there length. A karyotype is a chromosome pair chart. It is used often to identify abnormal numbers of chromosomes.

Did you that us humans have 23 pairs of chromosomes, or 46 chromosomes which ever you prefer. Did you now that some humans have whole are trisomic. Also did you now that trisomic can nondisjunction, which when something very wrong in meiosis in the earlier step which is bad.


 * __ Abnormal Numbers of Sex Chromosomes __**
 * There are many different abnormalities that can occur in the number of sex chromosomes. There might be an extra chromosome, or there might be some missing. This can cause problems.**
 * Any person that has at least one Y chromosome is a boy, and anyone without a Y chromosome is a girl. Usually, these people will turn out ok and have normal lives, but they may not be able to have children and some may have mental problems.**


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= Comprehension Check = 1.What is a pedigree? 2. Fruit flies can show examples of what? 3. What is Huntington's disease and what is it caused by? 4. What are the 23rd pair of chromosomes? 5. What are the other 22 pairs of chromosomes called? 6. What is incomplete dominance? 7. What is polygenic inheritance, and what is an example of it? 8. What are some examples of external influences? 9. What is an example of sex linked inheritance? 10. What is an effect of an abnormal number of sex chromosomes?

1. Pedigrees show the genetic inheritance of a certain trait 2. Fruit flies can show examples of sex-linked inheritance. 3. Huntington's disease is a lethal genetic disorder caused by a dominant allele 4. Sex Chromosomes X and Y 5. autosomes 6. Incomplete dominance follows mixing patterns, instead if inheriting one trait or the other you inherit a mix. 7. Polygenic inheritance is the inheritance pattern of a trait that is controlled by two or more genes. Such traits include skin color, a person’s height, and other traits that are controlled by many genes. 8. <span style="color: #000000; font-family: "Comic Sans MS",cursive;">temperature, nutrition, chemicals, light, and also infectious agents. <span style="color: #000000; font-family: "Comic Sans MS",cursive;"> 9. Color-blindness <span style="color: #000000; font-family: "Comic Sans MS",cursive;"> 10. They may not be able to have children, and they may have some mental problems.

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