Human Heredity 7 1 Chromosomes and Phenotype Two

Human Heredity

7. 1 Chromosomes and Phenotype Two copies of each autosomal gene affect phenotype. • Mendel studied autosomal gene traits, like hair texture.

7. 2 Complex Patterns of Inheritance The student is expected to: 6 F predict possible outcomes of various genetic combinations such as monohybrid crosses, dihybrid crosses and non-Mendelian inheritance TEKS 6 F

Beyond dominant and recessive alleles • There are some exceptions to Mendel’s principles. Luckily, none of these exceptions are exhibited in pea plants. • If so, Mendel would not have been able to figure out inheritance.

• Some alleles are neither dominant nor recessive. – Incomplete Dominance – Codominance

Incomplete dominance: • situation in which one allele is not completely dominant over another; the phenotype is a “blending” of the two alleles – Example: In some plants, when a truebreeding plant with red flowers is crossed with a true-breeding plant with white flowers, pink flowers are produced. Neither red nor white is dominant over the other.

7. 2 Complex Patterns of Inheritance Phenotype can depend on interactions of alleles. TEKS 6 F • In incomplete dominance, neither allele is completely dominant nor completely recessive. – Heterozygous phenotype is intermediate between the two homozygous phenotypes – Homozygous parental phenotypes not seen in F 1 offspring

• Consider this Punnett square:

Codominance: • situation in which both alleles of a gene contribute to the phenotype of the organism; both alleles are expressed but NOT blended – Example: In cows, the allele for red fur is codominant with the allele for white fur. Heterozygous cows carrying one red and one white allele have spotted fur, known as roan.

7. 2 Complex Patterns of Inheritance TEKS 6 F • Codominant alleles will both be completely expressed. – Codominant alleles are neither dominant nor recessive. – The ABO blood types result from codominant alleles. • Many genes have more than two alleles.

• Consider this Punnett square:

• Many traits are controlled by multiple alleles or multiple genes. – Multiple alleles (more than 2 choices) – Polygenic (multiple genes control a single trait)

Multiple alleles: • the case where three or more alleles of the same gene exist. Remember, an organism will have only two of these alleles (one from mom and one from dad). – Examples: Coat color in rabbits, blood type in humans

Multiple alleles:

Polygenic traits: • traits that are determined by alleles from more than one gene; these traits usually have a range of phenotypes – Examples: skin color in humans, height in humans

7. 2 Complex Patterns of Inheritance TEKS 6 F Many genes may interact to produce one trait. • Polygenic traits are produced by two or more genes. Order of dominance: brown > green > blue.

Human chromosomes 1. Most of our cells contain 23 pairs of chromosomes, for a total of 46 chromosomes. a. These cells are called somatic cells or body cells. b. Two exceptions are the gametes (sex cells), sperm and eggs, which have one copy of each chromosome. Sperm and egg cells each have 23 chromosomes.

Human chromosomes 2. There are two types of chromosomes. a. Autosomes: Of the 46 chromosomes, 44 of them (22 pairs of chromosomes) are called autosomes (non-sex chromosomes). b. Sex chromosomes: The last two chromosomes are called the sex chromosomes because they determine the sex of the person. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).

4. Scientists can analyze chromosomes by taking a picture of cells during mitosis. It is easiest to view chromosomes during mitosis because they are condensed. From a picture of chromosomes, scientists can cut and paste to arrange the chromosomes in pairs to form a karyotype. They are arranged from largest (pair #1) to smallest (pair #22). The last pair (#23) is the sex chromosomes.

Karyotype X Y

Male or Female? X X Y Male X Female

Karyotype Analysis Too many = Trisomy ___ Too few = monosomy ____ If there is a Y it is male, no matter how many X’s Only X it is a female

Human Traits 1. A pedigree is similar to a family tree- both are used to show relationships in a family. 2. Pedigrees can be used to demonstrate how traits are passed from one generation to another. 3. Genetic counselors use pedigrees to follow how genetic disorders are inherited. 4. People who are heterozygous for a recessive genetic disorder (they are unaffected) are called carriers.

female marriage male parents children

Add to your notes: Sometimes, but not always, carriers of traits (heterozygotes) may be represented as a half-shaded shape or a shape with a dot in the middle.

Human Genes: Blood typing In humans, blood type is determined by the Rh blood group and the ABO blood group. 1. The Rh blood group determines if your blood is positive or negative. a. There are two Rh alleles: the Rh+ allele is dominant to the Rh- allele. b. Your blood is positive if you are Rh+ /Rh+ or Rh+/Rh-. Your blood is negative if you are Rh-/Rh-

2. The ABO group is more complicated. There are three alleles: IA, IB, and i. a. The IA and IB alleles are codominant. The IA and IB alleles cause expression of carbohydrate chains called antigens on surface of red blood cells. They help your body identify the cells. b. The i allele is recessive to the other two alleles. The i allele O does not produce antigens.

c. The ABO blood group is important in blood transfusions. If the blood recipient has never been exposed to an antigen (A or B) and that antigen enters the body it will cause an immune reaction. This can cause death. Donors A A Recipients i. B AB O = cannot transfuse; immune reaction (clumping of cells) = OK to transfuse

ii. In emergency rooms when there isn’t time to figure out the blood type of the patient, which type of blood will the patient receive? Type O because these blood cells have no A or B antigens. People with Type O blood are called universal donors. iii. Who is the universal recipient that can receive blood from any donor? Type AB.

Human Chromosomes

Most genetic disorders are caused by mutations on autosomes, or non-sex chromosomes. Examples include: 1. Autosomal recessive disorders: albinism, cystic fibrosis, Tay-Sachs disease 2. Autosomal dominant disorders: most common form of dwarfism (achondroplasia) and Hypercholesterolemia (high cholesterol) 3. Codominant disorders: sickle-cell disease

Albinism Achondroplasia

Some genetic disorders are caused be genes on the sex chromosomes. 1. Most of these genes are on the X chromosome because the Y chromosome is very small and has few genes. The genes on X chromosome are different from the genes on the Y chromosome.

2. Because females are XX they have two copies of the genes on the X chromosome. For sex-linked traits, females can be homozygous dominant, heterozygous, or homozygous recessive.

7. 1 Chromosomes and Phenotype Males and females can differ in sex-linked traits. • Genes on sex chromosomes are called sex-linked genes. – Y chromosome genes in mammals are responsible for male characteristics. – X chromosome genes in mammals affect many traits.

7. 1 Chromosomes and Phenotype • Male mammals have an XY genotype. – All of a male’s sexlinked genes are expressed. – Males have no second copies of sex-linked genes.

7. 1 Chromosomes and Phenotype • 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.

3. Because males are XY they have only one copy of the genes on the X chromosome; this is called hemizygous. In males, only one recessive allele on the X chromosome is necessary for the recessive phenotype to be expressed because there is not another allele for this gene on the Y chromosome. Some sex-linked (also known as X-linked) genetic conditions include: a. Color blindness- the inability to distinguish certain colors b. Hemophilia- missing protein important for blood clotting c. Duchenne Muscular Dystrophy- progressive weakening of skeletal muscles

Colorblindness- Test A Everyone should be able to see a circle, star, and square in the demonstration picture.

Colorblindness- Test B Colorblind individuals should see the yellow square. Color normal individuals should see the yellow square and a "faint" brown circle.

Colorblindness- Test C Colorblind individuals should see nothing. Color normal individuals should see a "faint" brown boat.

Example of a sex-linked Punnett square: – XBXb (heterozygous female with normal vision) crossed to XBY (hemizygous male with normal vision) XBY XB XB Xb Xb Xb. Y

Chromosomal Disorders Remember that meiosis is the reductional cell division that divides one diploid cell to produce four haploid gametes (sex cells, sperm or egg). Normally gametes have one copy of each chromosome. 1. Sometimes chromosomes might not separate properly during meiosis; this is called nondisjunction. 2. If nondisjunction occurs, abnormal numbers of chromosomes (usually one is missing or there is an extra copy of one) are found in gametes and disorders of chromosomal numbers may result.

gametes

3. Trisomy: Some chromosomal disorders are caused by having three copies of one chromosome. This is called trisomy. In trisomies, the gamete of one parent donated two of one type of chromosome to the child and the gamete of the other parent donated one chromosome (like normal). 4. Monosomy: Chromosomal disorders characterized by missing one chromosome are called monosomies. In monosomies, the gamete of one parent donated one chromosome and the other did not donate any.

Some examples of chromosomal disorders resulting from nondisjunction: 1. Down syndrome- Trisomy 21 2. Klinefelter’s syndrome- XXY (male) 3. Turner’s syndrome- XO (female) Add to your notes: As long there is at least one Y chromsome, the karyotype is male.

Human Genome Project • The human genome project sequenced the human genome. Now the code is being analyzed and scientists are finding genes for many traits and genetic disorders. In gene therapy, a gene that has been mutated and does not work properly is replaced by a normal, working copy of the gene. Gene therapy is a work in progress.