The Human Heritage: Genes and Environment Chapter 2 The Human Heritage: Genes and Environment

Genetic Code Chromosome: A threadlike structure made up of genes 46 in all human cells, except sperm and ova which have 23 Deoxyribonucleic acid (DNA): A long double-stranded molecule that makes up chromosomes Genes: The segments on a DNA molecule that act as hereditary blueprints for the organism’s development Model of DNA

DNA Replication A strand of DNA replicates by splitting down the middle of the rungs of its ladderlike structure Each free base picks up a new complementary partner: Cytosine (C) pairs with guanine (G) Adenine (A) pairs with thymine (T)

Sexual Reproduction and Genetic Transmission Mitosis: A Process of Cell Replication Meiosis: A Source of Variability Sexual Determinism: A Case of Variability Mitosis is the process of cell division that generates all the cells of the body except the germ cells. During mitosis each chromosome in the cell replicates, producing a new chromosome identical to the first. the cell then splits, the chromosomes separating so that one of them goes to each new cell. Mitosis ensures that identical genetic information is maintained in the body cells over the life of the organism.

Mitosis Mitosis is the process of cell division that generates all the cells of the body except the germ cells.

Meiosis Meiosis is the process of cell division that generates the germ cells. Formation of sperm. As meiosis in the male begins, the chromosome pairs replicate and one member of each pair is contributed to each new cell. Each new cell then divides and the replicated chromosomes separate. The result is four sperm cells, each of which contains one member (or a copy) of each of the original pairs of chromosomes.(b) Formation of the ovum. Meiosis in the female differs slightly from meiosis in the male. When the first division occurs, the cytoplasm (the matter comprising most of the material of the cell) divides in such a way that the two resulting cells are unequal in size. The smaller of the two cells disintegrates. The large cell, the ovum, does not divide again unless it is fertilized. If fertilization occurs, the replicated chromosomes in the ovum separate into two new cells. Again the cytoplasm divides unequally, and the smaller of the resulting cells disintegrates. The 23 chromosomes of the larger cell fuse with the 23 chromosomes of the sperm to form the zygote with its 46 chromosomes.

Human X (above) and Y (below) chromosomes Human X (above) and Y (below) chromosomes. Note how much larger the X chromosome is. Males have both an X and a Y chromosome but females have two X chromosomes.

Sexual Determinism: A Case of Variability The Five Sexes Fausto-Sterling (1993) 1- Male = testes 2- Female = ovaries 3- Hermaphrodites = 1 testes & 1 ovary 4- Merms = Male pseudohermaphrodites i.e., testes & some aspect of female genitalia 5- Ferms = Female pseudohermaphrodites i.e., ovaries & some aspect of male genitalia

Genotype + Environment  Phenotype An Interaction Genotype + Environment  Phenotype Genotype: An individual’s genetic endowment Environment: The totality of conditions and circumstances that surround an individual Phenotype: The observable characteristics of an individual Physical and psychological traits, health, behavior

Laws of Genetic Inheritance Gregor Mendel (1822-1884): Garden peas Allele: The specific form of a gene coded for a particular trait Homozygous: Having inherited two genes of the same allelic form for a trait Heterozygous: Having inherited two genes of different allelic forms for a trait

Heterozygous Possibilities Dominant gene: The allele that is expressed when an individual possesses two different alleles for the same trait Recessive genes: The allele that is not expressed when an individual possesses two different alleles for the same trait Codominance: An outcome in which a trait that is determined by two alleles is different from the trait produced by either of the contributing alleles alone

Inheritance of alleles for blood type Inheritance of alleles for blood type. The alternative forms of a gene for blood type, inherited in various combinations from the parents, produce different blood phenotypes. The combination of one allele from each parent is the genotype, and the resulting blood type of the child is the phenotype. Homozygous- genes for same trait Heterozygous- 1 gene from 1 parent, 1 from another I.e., cleft & “non-cleft” Dominant allele- the one whose characteristics are expressed Recessive allele- the one whose characteristics are not expressed Child may be affected by both alleles and express something intermediate Codominance- Child may be affected by both alleles, but express something distinctive from either allele

Sex-Linked Traits Given that the X chromosome is much larger than the Y chromosome, most inherited sex-linked characteristics are carried on the X chromosome Males are more susceptible to genetic defects than are females If a daughter has a harmful recessive gene on one X chromosome, she will usually have a normal dominant gene on the other X chromosome to override it A son who inherits a harmful recessive gene on his X chromosome has no such complementary allele to override the recessive gene’s harmful effects

Genotype and Phenotype The Laws of Genetic Inheritance Sex-Linked Genetic Effects Examples: Red-green color blindness, hemophilia, muscular dystrophy, night blindness,Hypogammaglobulin (lack of production of anitbodies to fight bacteria), Vitamin D Resistance (Rickets)

Genes, The Organism, and the Environment Range of Reaction- all possible gene/env relationships compatible w/ organism’s continued life Canalization- certain characteristics typical of a species may be restricted to a narrow range despite wide variations in environmental conditions The Study of Genetic Influences on Human Behavior- heritability I.e., genetically influenced v. genetically caused Estimating Genetic Influence Through Kinship Studies- twin & adoption studies Beh’l geneticists study how genetic & environmental factors combine to produce individual differences in beh. & have found 3 principles: 1- Interactions betw organism & env must be studied in a broad, ecological framework bec variations in env can hv profound effects. Eg., Winchester & Himalayan rabbit 2- Interaction is a 2-way process eg. Songbirds cn sing if exposed to their species 3- Genetic factors often play a role in determining what environments indi inhabit & how they shape & select their own experiences Canalization---WADDINGTON (1947) Gottieb (1997) canalization can result from early dev’l experiences as well eg., Mallard duck following a call Heritability= refers toVariance due to genes (this is estimated)/ total variance

Mutation = an error in the process of gene replication that results in a change in the molecular structure of the DNA 1- Mutations disturb the well-integrated mechanisms of development, so understanding them can reveal the relationship betw genes and environment 2- If genetic abnormalities can be detected early, then one can find ways to ameliorate birth defects 3- Try to find ways to reduce the impact of abnormalities of children

Mutations and Genetic Abnormalities Sickle-Cell Anemia: An Example of Gene- Environmental Interaction Down Syndrome: A Chromosomal Error Sex-Linked Chromosomal Abnormalities Phenylketonuria: A Treatable Genetic Disease See p. 67

Down Syndrome: A Chromosomal Error Cause: More than 95% of children born with Down syndrome have 3 (vs. 2) copies of chromosome 21 Traits: Mentally/physically retarded; distinctive physical characteristics (e.g., slanting eyes, rather flat facial profile, ears lower than normal, short neck, protruding tongue, dental irregularities, small curved fingers, wide-spaced toes); more likely to die young Incidence: 1:1000 births (increases with mother’s age: 1:100 by age 40, 1:32 by age 45, 1:12 by age 49) Outlook: Supportive intervention that includes special education by concerned adults can markedly improve the intellectual functioning of some of these children

Down Syndrome: A Chromosomal Error Cause: More than 95% of children born with Down syndrome have 3 (vs. 2) copies of chromosome 21 Traits: Mentally/physically retarded; distinctive physical characteristics (e.g., slanting eyes, rather flat facial profile, ears lower than normal, short neck, protruding tongue, dental irregularities, small curved fingers, wide-spaced toes); more likely to die young Incidence: 1:1000 births (increases with mother’s age: 1:100 by age 40, 1:32 by age 45, 1:12 by age 49) Outlook: Supportive intervention that includes special education by concerned adults can markedly improve the intellectual functioning of some of these children

Sickle-Cell Anemia: Gene–Environment Interaction Cause: Recessive gene (victims are homozygous, but heterozygous subjects are also mildly affected) Traits: Abnormal blood cells cause circulatory problems (e.g., heart enlargement) and severe anemia Incidence: 8-9% of U.S. blacks Outlook: Crippling, but treatable with medication

Klinefelter Syndrome: A Sex- Linked Abnormality Cause: Extra X chromosome (i.e., XXY) Traits: Males fail to develop secondary sex characteristics (e.g., facial hair, voice change), are sterile, and most have speech and language problems Incidence: 1:900 males Outlook: May be treatable with testosterone replacement therapy

Prenatal Testing Ultrasound Amniocentesis Chorionic Villus Sampling

Ultrasound Ultrasound creates images of the fetus by using high-frequency sound waves to "see" inside your body. This ultrasound image reveals the normal development of healthy twins.

Amniocentesis During an amniocentesis, samples of amniotic fluid are removed using a needle and then tested for genetic abnormalities in the laboratory.

Chorionic Villus Sampling During CVS, samples of the cells that line the placenta, known as chorionic villi cells, are removed and tested for genetic abnormalities in the laboratory.

Survival Strategies: Culture Cultural artifacts, such as tools and clothing, greatly extend the range of environments in which human beings can reproduce and thrive Cultural knowledge, such as the creation of “hazard prevention strategies,” serves to protect and support children to enable them to mature and reproduce