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Chapter 23 Heredity: The Continuity of Life Heredity • God has provided for all things to reproduce after their kind. • Characteristics are passed from parent to offspring. • This transmission of characteristics takes place through either sexual or asexual reproduction. Asexual Reproduction • Asexual reproduction is God’s provision from one generation to the next that requires on mitotic division. • Example: A bud of a sponge breaks off and grows a whole new sponge. • An amoeba divides into two amoebas. Differentiation of Cells • A fertilized egg undergoes mitosis and becomes many cells that are the same. • The many like cells then “differentiate” and become other kinds of cells…like a nerve cell or a blood cell. • The many different cells work together to be an organism! Sexual Reproduction • In organisms that reproduce sexually, the heredity of both parents is combined to provide the heredity of the offspring. • Male and female contribute equal amounts of genetic material. • In Humans each cell has 46 chromosomes… • However, the gametes (egg and sperm) contain only 23 chromosomes. Sexual Reproduction • This is because the pairs of chromosomes separate to make the gametes. • Plants also have egg cells (in ovaries) and sperm cells in anthers in the form of pollen. • When fertilization occurs (egg and sperm coming together) the result is a zygote. Sexual Reproduction • The sperm and egg each having half the chromosomes are said to be haploid. • The zygote having both sets of chromosomes and thus having 46 (for humans) is said to be diploid. • The single cell zygote now divides by mitosis to grow into the new organism. Classical Genetics • Gregor Mendel Austrian monk who grew pea plants… • He worked in 1853 and he noticed that traits were somehow passed on from one generation to the next. • He discovered that his plants bred true for each characteristic. Classical Genetics • For example: red blooms always produced red blooms. • For example: wrinkled seeds always produced plants with wrinkled seeds. • Mendel would say these plants were “true” for these traits – they were pure breeding lines. Classical Genetics • The reason pea plants worked so well for genetic research is because he could produce (grow) a lot of plants in a short period of time. Classical Genetics Classical Genetics • Mendel would cross pure bred lines for wrinkled seeds with pure bred lines for round seeds. • He would call this first generation of offspring the first filial generation (F1). Classical Genetics • This F1 generation would have all round seeds! • What happened to the wrinkled seed genes?! The Law of Dominance • Mendel concluded that traits are paired. • Each parent contributes one gene for each trait. The Law of Dominance • If both genes for the trait are the same in the offspring that offspring is homozygous for that trait. • If the genes are different the offspring is heterozygous for that trait. The Law of Dominance • Dominant genes for a trait show up even if the offspring is heterozygous. • A dominant gene will “mask” or hide the recessive trait. • In modern genetics, different forms of a gene for a particular trait are called alleles. Hybrids and Hybridization • Hybrid is used in genetics to describe an organism with two different traits. • A hybrid is not a pure strain. Punnett Squares • Geneticist use a Punnett Square to represent crosses between traits. Incomplete Dominance • When traits blend and neither one is dominant it is called incomplete dominance. • The traits mix for offspring that are something more of a mix. Incomplete Dominance • For example: a red pea blossom plant and a white pea blossom plant will combine to have offspring that are pink. • Neither trait is hidden. The Law of Segregation • Mendel found that if he took the F1 generation plants and crossed them with each other then the recessive trait would show up again. Law of Segregation • He determined that the recessive traits segregate or separate in some of the offspring. The Law of Segregation • He called this generation the F2 generation. The Law of Independent Assortment • An organism that is a hybrid for two traits is called a dihybrid. • These two traits will segregate or separate independent of each other. The Law of Independent Assortment • Sometimes traits (genes) are located on the same chromosome. • In this case, the traits will not segregate. • These traits are called linkage groups. • Mostly these are associated with genes on the sex chromosomes… X and Y Genetic Research in the 20th Century • Mendel’s work was neglected until 1900. • In 1902, Walter Sutton theorized that Mendel’s “heredity factors” were particles carried on chromosomes. Genetic Research in the 20th Century • Sutton explained that Mendel’s round/wrinkled and green/yellow seed traits were on separate chromosomes. • His theories laid the foundation for all modern genetic research. Genetic Research in the 20th Century • In 1904, Thomas Morgan began research that would earn him the Nobel prize in 1933. • Morgan began to study fruit flies (Drosophila melanogaster)… Genetic Research in the 20th Century • Morgan found a pair of non-identical chromosomes. • He determined that these pairs are the sex chromosomes. • The female had identical chromosomes he named the X chromosomes. • The male had one chromosome like the female (he called it X) and one different (he called it Y). Genetic Research in the 20th Century • In most organisms it is the sperm that determines the sex of the offspring. Sex-Linked Traits • Morgan discovered in 1909 that some traits are carried on the sex chromosomes. • He called these sex-linked – they are only carried on the X chromosome. • A male who inherits a gene with a particular trait that is carried on the X chromosome will always display that trait since he only needs one gene to display it. Human Genetics • The Human Genome Project begun in the 1990’s has completely mapped all the human genes. • Dominant and recessive genes are expressed by lower case and upper case letters. • These letters describe the genotype of the organism. • The expression of these genes is the phenotype (what the organism looks like) Human Genetics Dominant Gene Inheritance • There are many genes in the human population that are dominant. • The gene for free ear lobe attachment is dominant. Human Genetics Dominant Gene Inheritance • Cystic fibrosis is a recessive trait. Human Genetics Incomplete Dominance • Sickle cell anemia is an incomplete dominant gene. Human Genetics Multiple Gene Inheritance • Multiple gene inheritance occurs when there are than two alleles (forms) of a gene for a trait. • One example is blood type. • A, B, AB, and O are the four possible blood types in the human population. • A person with IA and IB as the two genes has blood type AB. Human Genetics Multiple Gene Inheritance • This means the person has proteins A and B on their blood cells. • A person with IoIo has blood type O and has no protein markers on their blood cells. Human Genetics Pleiotrophy Inheritance • Pleiotrophy is when a gene affects several other genes that are seemingly unrelated. Human Genetics Polygenic Inheritance • Polygenic inheritance is when traits are controlled by different pairs of genes. • Skin color, for example, is controlled by as many as 4 pairs of genes. • Height is controlled by as many as 4 pairs of genes. Sex-Linked Characteristics • Some characteristics are carried on the X chromosome. • Hemophilia is a disease carried on the X. • Hemophilia is a blood disorder that causes the blood not to clot. • Colorblindness is another X gene disorder. • A female carrier will have the genes XcX. • A female with color blindness will have the genes XcXc. Genetic Advances • We now can detect genetic disorders at birth. • Some genetic advances in early detection of disease have given parents the “option” of aborting a baby who will be born with a disorder as opposed to letting them be born alive. • In the United States, 9 out of 10 children with down syndrome are aborted. • Some people have even advocated the practice of eugenics – the control of human population through genetics. • This would involve “selecting” superior individuals for mating only or sterilizing those deemed “inferior”.