Slide 1 of 43 Biology Mr. Karns Human Heredity Slide 2 of 43 141 Human Heredity 14-1 Human Heredity 141 Human Heredity Slide 3 of 43 Human Chromosomes Cell biologists analyze chromosomes by looking at karyotypes. Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs. A picture of chromosomes arranged in this way is known as a karyotype. 141 Human Heredity Slide 4 of 43 Human Chromosomes Human Karyotype 141 Human Heredity Slide 5 of 43 Human Chromosomes Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex. Females have two copies of an X chromosome. Males have one X chromosome and one Y chromosome. The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes. 141 Human Heredity Slide 6 of 43 Human Chromosomes How is sex determined? 141 Human Heredity Slide 7 of 43 Human Chromosomes All human egg cells carry a single X chromosome (23,X). Half of all sperm cells carry an X chromosome (23,X) and half carry a Y chromosome (23,Y). About half of the zygotes will be 46,XX (female) and half will be 46,XY (male). 141 Human Heredity Slide 8 of 43 Human Chromosomes Males and females are born in a roughly 50 : 50 ratio because of the way in which sex chromosomes segregate during meiosis. 141 Human Heredity Slide 9 of 43 Human Traits In order to apply Mendelian genetics to humans, biologists must identify an inherited trait controlled by a single gene. They must establish that the trait is inherited and not the result of environmental influences. They have to study how the trait is passed from one generation to the next. Slide 10 of 43 141 Human HeredityHuman Traits Pedigree Charts A pedigree chart shows the relationships within a family. Genetic counselors analyze pedigree charts to infer the genotypes of family members. 141 Human Heredity Slide 11 of 43 Human Traits A circle represents a female. A horizontal line connecting a male and a female represents a marriage. A shaded circle or square indicates that a person expresses the trait. A square represents a male. A vertical line and a bracket connect the parents to their children. A circle or square that is not shaded indicates that a person does not express the trait. 141 Human Heredity Slide 12 of 43 Human Traits Genes and the Environment Some obvious human traits are almost impossible to associate with single genes. Traits, such as the shape of your eyes or ears, are polygenic, meaning they are controlled by many genes. Many of your personal traits are only partly governed by genetics. 141 Human Heredity Slide 13 of 43 Human Genes The human genome includes tens of thousands of genes. In 2003, the DNA sequence of the human genome was published. In a few cases, biologists were able to identify genes that directly control a single human trait such as blood type. 141 Human Heredity Slide 14 of 43 Human Genes Blood Group Genes Human blood comes in a variety of genetically determined blood groups. A number of genes are responsible for human blood groups. The best known are the ABO blood groups and the Rh blood groups. 141 Human Heredity Slide 15 of 43 Human Genes The Rh blood group is determined by a single gene with two allelespositive and negative. The positive (Rh + ) allele is dominant, so individuals who are Rh + /Rh + or Rh + /Rh are said to be Rh- positive. Individuals with two Rh - alleles are said to be Rh- negative. 141 Human Heredity Slide 16 of 43 Human Genes ABO blood group There are three alleles for this gene, I A, I B, and i. Alleles I A and I B are codominant. 141 Human Heredity Slide 17 of 43 Human Genes Individuals with alleles I A and I B produce both A and B antigens, making them blood type AB. 141 Human Heredity Slide 18 of 43 Human Genes The i allele is recessive. Individuals with alleles I A I A or I A i produce only the A antigen, making them blood type A. 141 Human Heredity Slide 19 of 43 Human Genes Individuals with I B I B or I B i alleles are type B. 141 Human Heredity Slide 20 of 43 Human Genes Individuals who are homozygous for the i allele (ii) produce no antigen and are said to have blood type O. 141 Human Heredity Slide 21 of 43 Human Genes Recessive Alleles The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype. Many disorders are caused by autosomal recessive alleles. 141 Human Heredity Slide 22 of 43 Human Genes 141 Human Heredity Slide 23 of 43 Human Genes Dominant Alleles The effects of a dominant allele are expressed even when the recessive allele is present. Two examples of genetic disorders caused by autosomal dominant alleles are achondroplasia and Huntington disease. 141 Human Heredity Slide 24 of 43 Human Genes 141 Human Heredity Slide 25 of 43 Human Genes Codominant Alleles Sickle cell disease is a serious disorder caused by a codominant allele. Sickle cell is found in about 1 out of 500 African Americans. 141 Human Heredity Slide 26 of 43 Human Genes 141 Human Heredity Slide 27 of 43 From Gene to Molecule How do small changes in DNA cause genetic disorders? 141 Human Heredity Slide 28 of 43 From Gene to Molecule In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder. 141 Human Heredity Slide 29 of 43 From Gene to Molecule Cystic Fibrosis Cystic fibrosis is caused by a recessive allele. Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways. 141 Human Heredity Slide 30 of 43 From Gene to Molecule The most common allele that causes cystic fibrosis is missing 3 DNA bases. As a result, the amino acid phenylalanine is missing from the CFTR protein. 141 Human Heredity Slide 31 of 43 From Gene to Molecule Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane. 141 Human Heredity Slide 32 of 43 From Gene to Molecule The cells in the persons airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus. 141 Human Heredity Slide 33 of 43 From Gene to Molecule Sickle Cell Disease Sickle cell disease is a common genetic disorder found in African Americans. It is characterized by the bent and twisted shape of the red blood cells. 141 Human Heredity Slide 34 of 43 From Gene to Molecule Hemoglobin is the protein in red blood cells that carries oxygen. In the sickle cell allele, just one DNA base is changed. As a result, the abnormal hemoglobin is less soluble than normal hemoglobin. Low oxygen levels cause some red blood cells to become sickle shaped. 141 Human Heredity Slide 35 of 43 From Gene to Molecule People who are heterozygous for the sickle cell allele are generally healthy and they are resistant to malaria. 141 Human Heredity Slide 36 of 43 From Gene to Molecule There are three phenotypes associated with the sickle cell gene. An individual with both normal and sickle cell alleles has a different phenotyperesistance to malaria from someone with only normal alleles. Sickle cell alleles are thought to be codominant. 141 Human Heredity Slide 37 of 43 From Gene to Molecule Malaria and the Sickle Cell Allele Regions where malaria is common Regions where the sickle cell allele is common - or - Continue to: Click to Launch: Slide 38 of 43 141 End Show Slide 39 of 43 141 A chromosome that is not a sex chromosome is know as a(an) a.autosome. b.karyotype. c.pedigree. d.chromatid. End Show Slide 40 of 43 141 Whether a human will be a male or a female is determined by which a.sex chromosome is in the egg cell. b.autosomes are in the egg cell. c.sex chromosome is in the sperm cell. d.autosomes are in the sperm cell. End Show Slide 41 of 43 141 Mendelian inheritance in humans is typically studied by a.making inferences from family pedigrees. b.carrying out carefully controlled crosses. c.observing the phenotypes of individual humans. d.observing inheritance patterns in other animals. End Show Slide 42 of 43 141 An individual with a blood type phenotype of O can receive blood from an individual with the phenotype a.O. b.A. c.AB. d.B. End Show Slide 43 of 43 141 The ABO blood group is made up of a.two alleles. b.three alleles. c.identical alleles. d.dominant alleles. END OF SECTION