Human Genetics Patterns of Inheritance for Human Traits

Human Genetics: Patterns of Inheritance for Human Traits

Terms to know � Gene – segment of DNA. Carries genetic information that codes for a trait. � Chromosome - compact packages of DNA. Inside nucleus. � Sperm – male sex cell (gamete) � Egg – female sex cell (gamete)

Terms to know � Karyotype A picture. � Autosome – arrangement of chromosomes. - any chromosome that is not a sex chromosome

Terms to know: �Genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype. ◦ A capital letter represents the dominant form of a gene (allele) ◦ A lowercase letter is the abbreviation for the recessive form of the gene (allele). Example: V = dominant v = recessive

Terms to know �Dominant: a gene that is expressed, regardless of whether its counterpart allele on the other chromosome is dominant or recessive. ◦ Ex: VV or Vv = Long wings �Recessive: gene that produces its characteristic phenotype only when its allele is identical; vv= short wings

Terms to know: �Phenotype = the physical appearance of a trait in an organism ◦ BB, Bb = brown eyes ◦ bb = blue eyes

Terms to know �Allele: different versions of the same gene occupying a given position on a chromosome Chromosome from Mom Chromosome from Dad

Terms to know � Organisms that have 2 identical alleles for a particular trait are called homozygous (TT or tt). � Organisms that have 2 different alleles for a particular trait are called heterozygous (Tt).

What makes us human? � The use of a microscope to analyze cells during mitosis shows condensed chromosomes � Scientists cut pictures of these chromosomes out and arrange them into a karyotype (right). =The number and visual appearance of the chromosomes in the cell nuclei of an organism or species in

The beginning � 2 haploid gametes (egg and sperm) join together during fertilization. � The result is a fertilized egg called a zygote which is diploid. � Humans have 46 chromosomes

Male or Female? � Two of the 46 chromosomes are sex chromosomes because they determine whether the zygote will develop into a male or female � Females � Males have XX chromosomes have XY chromosomes

What about the other chromosomes? � The 44 chromosomes that are not sex chromosomes are called autosomes.

Will it be a boy or a girl? � Because of the way sex chromosomes segregate, there is a 50: 50 chance of both males and females being born � All human egg cells carry a single X chromosome, but only half of sperm cells carry an X while the other half carries a Y Which parent determines the sex of the baby?

Human Traits � To identify an inherited trait controlled by a single gene, first we need to determine if that the trait is inherited and not a result of environmental influences. Then, we have to study how it’s passed from one generation to the next

Pedigrees �A pedigree chart shows the relationships within a family and how particular traits are passed from one generation to the next

So can you determine who gave you those good looks? � Nope. Unfortunately, many traits are polygenic, meaning they are controlled by many genes. ◦ Example: eye or ear shape � Additionally many traits are influenced by the environment ◦ Example: genetics determines a person’s maximum possible height, nutrition also plays a role in whether or not that height is ever reached

Important Note: � While it’s important to consider environmental effects on expression of genes… � Environmental effects on genes expression ARE NOT INHERITED!!! ◦ Analogy: Just because someone’s arm gets cut off doesn’t mean their child will be born missing an arm!

YOUR TURN! Determine whether albinism is a dominant or recessive trait: aa Aa Aa Aa aa Recessive

Dominance � Dominant alleles will be expressed even if the recessive allele is present � Codominance: both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other R = Red flower W = White flower RW = Red & White spotted flowers

Incomplete Dominance � Incomplete Dominance: both alleles of a gene are expressed in the phenotype- resulting in combined phenotype red flowers crossed with white flowers = pink flowers

How can you tell the difference? � The trick is to recognize when you are dealing with a question involving incomplete dominance. � There are two steps to this: 1) Notice that the offspring is showing a 3 rd phenotype. The parents each have one, and the offspring are different from the parents. 2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

Chromosomal Disorders � When meiosis goes wrong and homologous chromosomes fail to separate its called nondisjunction. This means “not coming apart. ” � When this happens abnormal numbers of chromosomes end up in gametes and a disorder of chromosome numbers may result. ◦ Examples: Down Syndrome, Turner’s Syndrome, Klinefelter’s Syndrome

Down Syndrome � If 2 copies of an autosomal chromosome fail to separate in meiosis an individual may be born with 3 copies of a chromosome. This is known as “trisomy” � Down Syndrome is the most common form of this, called Trisomy 21.

Child with Downs Syndrome • The extra chromosome causes problems with the way the body and brain develop.

Sex Chromosome Disorders � In females, nondisjunction can lead to Turner’s Syndrome ◦ She inherits only 1 X (45, X) ◦ Sterile and sex organs don’t develop at puberty � In males, nondisjunction can lead to Klinefelter’s Syndrome (47, XXY) ◦ Extra X interferes with meiosis and usually prevents individuals from reproducing ◦ Estimated 1 in every 500 -600 males ◦ XXXY and XXXXY have been found!

So what happens if a gene sucks? � The Human Genome Project may help in finding cures for genetic disorders through gene therapy. � Gene therapy is the process of changing a gene that causes a genetic disorder. An absent or faulty gene is replaced by a normal, working gene.

Gene Therapy cont…. � The most common form of gene therapy involves using DNA that encodes a functional, therapeutic gene in order to replace a mutated gene. � Today, most gene therapy studies are aimed at cancer and hereditary diseases linked to a genetic defect

Ethically Speaking � If scientists can find ways to cure genetic diseases is it okay if they also start engineering taller people, or their hair color, skin color, sex, blood group or appearance? ? ? � What are the kinds of ramifications (effects) gene therapy in this way could have on human populations in the world?

What about lacking an X? � There have been NO incidences in which an individual lacks an X chromosome all together � This suggests that the X chromosome contains genes that are vital for the survival and development of an embryo

5 Patterns of Inheritance for Human Traits 1. Single Allele Dominant 2. Single Allele Recessive 3. Sex Linked (X-Linked) 4. Multiple Alleles 5. Polygenic Traits

Single Allele Genes � 1. Regular traits that are either determined by a dominant or recessive allele on an autosome Autosomal Dominant examples: a. Huntington’s Disease b. Achondroplasia (dwarfisim) c. Polydactyly (extra fingers and toes) 2. Autosomal Recessive examples: a. Albinism b. Cystic Fibrosis c. Sickle Cell Anemia

1. Dominant Allele Disorders Huntington’s Disease (HD) § Results in loss of muscle control and mental deterioration § No signs are shown until 30’s § Brain degeneration § Treatment: No cure, but drug treatments are available to help manage symptoms.

1. Dominant Allele Disorders Achondroplasia § § Dwarfism Person grows no taller than 4’ 4

1. Dominant Allele Disorders Polydactyly § The presence of more than the normal number of fingers or toes. § Can usually be corrected by surgery.

2. Recessive Allele Disorders Albinism § Lack of pigment in skin, hair, and eyes § Mutation in one of several genes which provide the instructions for producing one of several proteins in charge of making melanin.

2. Recessive Allele Disorders Cystic Fibrosis (CF) § Caused by recessive allele on chromosome 7 § Small genetic change (removes one Amino Acid) changes protein § Results in: Excess mucus in the lungs, liver and digestive tract, gets infection easily, and early death unless treated.

2. Recessive Allele Disorders Sickle Cell Disease § Red blood cells are bent and twisted § Get stuck in capillaries damage tissues § Results in weakness, damage to brain and heart

3. Sex Link Traits: Genes on the X and Y chromosomes

how we determine gender… How do we determine the sex of an individual? through sex chromosomes § Sex Chromosomes: X and Y Female: XX Male: XY § Who is responsible for gender determination in the child? The father because he gives an X or Y to the gametes. The mother only gives an X to the gamete § Which sex chromosome is bigger? the X chromosome (y is much smaller)

What are sex-linked genes? genes found on a sex chromosome � X-linked genes are genes found on the X chromosome, symbolized by Xr, XR, Y 0. � Y-linked genes are found on the Y chromosome, symbolized by X 0, YR, Yr � Thomas Morgan experimented with the eye color of fruit flies (Drosophilia) to determine Xlinkage

Examples of X-linked traits: 1. 2. 3. 4. Color Blindness Muscular Dystrophy Hemophilia Icthyosis simplex (scaly skin)

Sex-Linked Genes � Turns out, there’s a special pattern of inheritance for genes located on the X or Y chromosome. � Because these chromosomes determine sex, genes located on them are termed sex-linked

Colorblindness � About 1 in 10 males is color blind, while 1 in 100 females are color blind in the U. S. � Colorblindness is X-linked. This means that the genes for color vision are located on the X chromosome. � Since females have 2 Xs they have a better shot at color vision, while males only have 1 X which means all X-linked alleles are expressed even if they are recessive.

Colorblindness

Colorblindness �A person with normal color vision sees a number seven in the circle above. � Those at all. who are color blind usually do not see any number

Colorblindness � RED-GREEN COLORBLINDNESS: � People with red-green color blindness see either a three or nothing at all. � Those with normal color vision see an 8.

Duchenne Muscular Dystrophy � Sex-linked � Results in progressive weakening and loss skeletal muscle � 1 in 3, 000 U. S. males is born with it of

What happens to the extra X in females? � Males only get one copy of the X chromosome, so what happens to the extra one in females? � One X is randomly turned “off”. This is called X-Chromosome Inactivation. It forms a dense region in the nucleus called a Barr Body

This happens in cats! � One X chromosome may have the allele for orange spots while the other X may have the allele for black spots � In cells in different parts of the body one X may be turned on while the other X is turned on elsewhere � The cat’s fur will have a mixture of orange and black spots � Fact: If a cat has 3 colors, it’s female. Males can only have 2 colors (unless they have genetic disorder)!

Hemophilia- Lacking in the ability to clot blood ◦ There is a gene on the “X” chromosome that controls blood clotting ◦ People who have hemophilia are missing the protein to clot blood ◦ They can bleed to death by a minor cut.

Muscular Dystrophy - Results in weakening/loss of muscles - Caused by defective version of gene that codes for muscle

Sex-influenced Traits � Sex-Influenced traits are those that are on autosomes, but occur because of the sex hormones in male and female bodies. � Examples: Facial hair Baldness

4. Multiple Alleles – any gene that has 3 or more alleles (not just 1 dominant and 1 recessive) Example: Blood type has 3 alleles: IA= Type A blood (dominant) IB= Type B blood (dominant) i = Type O blood (recessive)

Check out the possible genotypes and phenotypes of blood below: Genotypes Phenotypes IAIA or IAi Type A IBIB or IBi Type B I AI B Type AB ii Type O

Blood has both Multiple Alleles and is Co. Dominant � If you have IAIB as your genes, you have both Type A and Type B blood, also known as Type AB � If you have IAi, i is recessive IA, so you have type A blood � Q. When would you have Type O blood? � A. When you have ii as your genotype. to

5. Polygenic Traits – traits controlled by 2 or more genes that interact, forming the trait § § Usually show a wide range of phenotypes Ex: Skin color, eye color, foot size, height �Wide range of skin colors because there are more than 4 genes that control this trait. § These may also be influenced by the environment, for example height. If not given the proper nutrition as a child, they might not be as tall as their genes dictate.