CHAPTER 2

1. CHAPTER 2 Heredity and Conception

2. Learning Outcomes LO1 Explain the influences of heredity on development, referring to chromosomes and genes, mitosis and meiosis, twins, and dominant and recessive traits. LO2 Describe the features and causes of various chromosomal abnormalities. LO3 Describe the features and causes of various genetic abnormalities. LO4 Discuss methods of detecting genetic abnormalities. LO5 Describe methods of determining our genotypes and our phenotypes. LO6 Describe the process of conception. LO7 Discuss the causes of infertility and alternate ways of becoming parents. © Fancy/Veer/Corbis

3. TRUTH OR FICTION? • T F Your father determined whether you are female or male. • T F You can carry the genes for a deadly illness and not become sick yourself. • T F Approximately 120 to 150 boys are conceived for every 100 girls. • T F Sperm travel about at random inside the woman’s reproductive tract. • T F “Test-tube” babies are grown in a laboratory dish throughout their 9- month gestation period. • T F You can select the sex of your child. © iStockphoto.com

4. LO1 The Influence of Heredity on Development © Fancy/Veer/Corbis

5. The Influence of Heredity on Development • Heredity defines our nature. • It is based on the biological transmission of physical and psychological traits from generation to generation. • The field of biology that studies heredity is called GENETICS. • Traits are transmitted by: • Chromosomes • Genes

6. Chromosomes and Genes • CHROMOSOMES • Rod-shaped structures found in cells • Typical human cells have 46 (23 pairs) • Each contains thousands of segments called genes. • GENES • Biochemical materials that regulate how traits develop • Some traits are transmitted by a single pair of genes. • Other traits are POLYGENIC (using several pairs). • We (humans) have approximately 20-25,000 genes. • Genes are segments of strands of deoxyribonucleic acid (DNA). • DNA forms a double spiral (HELIX) that looks like a twisting ladder. • Each “rung” on the ladder consist of 4 basic chemicals that are placed in pairs of either: • Adenine and Thymine • Cytosine and Guanine • The sequence of the rungs are the individual genetic code that cause the developing organism to grow arms, wings, skin, or scales.

7. Figure 2.1 – The Double Helix of DNA

8. Beginnings of Life – Mitosis & Meiosis • Life begins as a single cell or zygote that divides repeatedly. • There are TWO types of cell division. • Mitosis and Meiosis

9. Mitosis • Mitosis: genetic code carried into new cells in our bodies • DNA breaks apart (unzips); and the double helix duplicates. • DNA forms two camps on either side of cell; cell divides. Each incomplete rung combines with its partner to form a new ladder; resulting identical copies of the DNA strand separate when cell divides; each is newly formed cell; genetic code is identical in new cells unless mutations occur through environmental influences such as radiation; mutations occur by chance

10. Figure 2.2 – The Double Helix of DNA

11. Meiosis • Sperm and ova are produced through meiosis or reduction division. • 46 chromosomes within the cell nucleus first line up into 23 pairs. • DNA ladders unzip, leaving unpaired halves of chromosome; when cell divides each member of each pair goes to each newly formed cell. • Each new cell nucleus contains only 23 chromosomes, not 46. • 23 chromosomes come from the mother and 23 chromosomes come from the father; 22 pairs are autosomes and 23rd pair are sex chromosomes.

12. Identical and Fraternal Twins • Sometimes a zygote divides into two cells that separate and becomes two individuals or twins. There are two types of twins: • Monozygotic (MZ): zygote divides into two cells that separate so each develops into individual with same genetic makeup - IDENTICAL • Dizygotic (DZ): two ova are produced in the same month and they are each fertilized by a different sperm cell - FRATERNAL • DZ runs in families through the maternal side; if mother or grandmother was a twin, or mother has had twins already the chances increase of her bearing twins again • Ovulation: as a woman nears end of child-bearing years, ovulation becomes less regular causing multiple ovum to be released some months, thus increasing likelihood of twins • Fertility Drugs also increase the chances of multiple births by causing more than one ovum to be released.

13. Dominant and Recessive Traits • Traits are determined by pairs of genes; each member of a pair of genes is termed an allele. • Homozygous: both of the alleles for a trait are the same • Heterozygous: alleles for a trait differ • Incomplete dominance/codominance: effects of both alleles are shown - a sort of “averaging” • Dominant trait: trait whose influence will be shown each time the gene is present • Recessive trait: trait whose influence will be shown only when it is paired with a second recessive gene • Carriers: people who bear one dominant gene and one recessive trait

14. Figure 2.4 – Transmission of Dominant and Recessive Traits

15. Figure 2.1 – Examples of Dominant and Recessive Traits

16. LO2 Chromosomal Abnormalities © Fancy/Veer/Corbis

17. Chromosomal Abnormalities • Chromosomal or genetic abnormalities can cause health problems. • Some disorders are caused by abnormalities in the 22 pairs of autosomes. • Others are found in the 23rd pair (sex chromosomes). • Some genetic abnormalities are caused by combinations of genes called: • Multifactorial problems • These reflect both a predisposition AND environmental contributors.

18. Down Syndrome • Caused by an extra chromosome of the 21st pair, resulting in 47 chromosomes. • Probability of having a Down Syndrome child increases with advancing age of parents. • Characteristic features: • rounded face, protruding tongue, broad, flat nose, sloping fold of skin over the inner corners of the eyes • Show deficits in cognitive and motor development • Typically die from cardiovascular problems by middle age, although modern medicine has extended life expectancy.

19. Figure 2.5 – Down Syndrome The development and adjustment of children with Down syndrome are related to their acceptance by their families. Children with Down syndrome who are reared at home develop more rapidly and achieve higher levels of functioning than those who are reared in institutions. © U.P. Images/iStockphoto.com / © Tomasz Markowski/iStockphoto.com

20. Sex-Linked Chromosomal Abnormalities • Sex-linked chromosomal abnormalities: disorders stemming from abnormal number of sex chromosomes • Most individuals with disorder are infertile. • Approx. 1 male in 700/1000 has extra Y chromosome resulting in heightened male secondary sex characteristics

21. Sex-Linked Chromosomal Abnormalities • XYY’s somewhat taller and develop heavier beards. • Once termed “supermales” due to the characteristics. • But often have more problems than XY’s • Such as mild delays in language development

22. Klinefelter Syndrome (XXY) • 1 male in 500 has syndrome • Caused by extra X sex chromosome • Produces less testosterone than normal males • Therefore, the testes, deepening of voice, musculature, and male pattern of body hair does not develop properly. • Usually have enlarged breasts (gynecomastia) • Typically mildly retarded, particularly language • Treated with testosterone replacement therapy, they see improvement in sex characteristics and mood elevation but remain infertile.

23. Turner Syndrome (X) • Approx. 1 girl in 2,500 has syndrome • Female has single X chromosome. • External genitals are normal, ovaries poorly developed, producing little estrogen • Shorter than average and infertile • Cognitive deficits with low estrogen: problems with visual-spatial skills, mathematics, and nonverbal memory

24. Triple X Syndrome (XXX) • Approx. 1 girl in 1,000 has syndrome • Normal in appearance • Tend to show lower-than-average language skills • Poorer memory for recent events • Development of external sex organs appear normal • Increased incidence of infertility

25. LO3 Genetic Abnormalities © Fancy/Veer/Corbis

26. Genetic Abnormalities • A number of disorders attributed to genes: • Phenylketonuria (PKU) • Huntington’s Disease • Sickle-Cell Anemia • Tay-Sachs Disease • Cystic Fibrosis • Sex-Linked Genetic Abnormalities

27. Phenylketonuria (PKU) • Enzyme disorder transmitted by a recessive gene affecting 1 child in 8,000. • Cannot metabolize an amino acid called phenylalanine; builds up in body and impairs functioning of the central nervous system (CNS) • Results are mental retardation, psychological disorders, physical problems • No cure, but PKU can be detected in new born children through blood or urine analysis; if identified placed on diets low in phenylalanine within three weeks of birth and develop normally

28. Huntington’s Disease • Affects approx. 1 in 18,000 Americans • Fatal, progressive degenerative disorder • Dominant trait • Physical symptoms include uncontrollable muscle movements • Psychological symptoms include loss of intellectual functioning and personality change • Onset during middle adulthood • Half of their offspring will have disorder • Affects 1 in 18,000 Americans • No cure, but helpful medicines

29. Sickle-Cell Anemia • Caused by a recessive gene • Most common among African Americans • 1 in 10 African Americans • 1 in 20 Latino/a Americans • Red blood cells take on the shape of a sickle and clump together, obstructing blood and oxygen supply. • Decreased oxygen can impair cognitive and academic functions. • Physical problems include: painful swollen joints, jaundice, and potentially fatal conditions such as pneumonia, stroke, and heart and kidney failure

30. Tay-Sachs Disease • Caused by recessive gene • Causes CNS to degenerate resulting in death • Commonly found among children in Jewish families of Eastern European background • 1 in 30 Jewish Americans carry recessive gene • Children with disorder progressively lose control of muscles experiencing visual and auditory sensory losses, develop mental retardation, become paralyzed, and die by end of early childhood (age 5).

31. Cystic Fibrosis • Caused by recessive gene • Most common fatal hereditary disease among European Americans • Approx. 30,000 Americans have disorder, 10 million more are carriers (1 in 31 people) • Children suffer from excessive production of thick mucus that clogs the pancreas and lungs. • Most victims die of respiratory infections in their 20s.

32. Sex-Linked Genetic Abnormalities • Genetic defects only carried on the X sex chromosome • Hemophilia: inability of the blood to clot • Queen Victoria was a carrier of hemophilia © Photo 12/The Image Works

33. Sex-Linked Genetic Abnormalities • Duchenne Muscular Dystrophy: weakening of muscles, inability to walk, general wasting away, and sometimes death • Involve recessive genes • Females with two X sex chromosomes are less likely to show sex-linked disorder. • Sons of female carriers are more likely to be afflicted.

34. LO4 Genetic Counseling and Prenatal Testing © Fancy/Veer/Corbis

35. Genetic Counseling and Prenatal Testing • Genetic counselors compile information about a couple’s genetic heritage to explore if their children will have a genetic abnormality. • Couples with likelihood of passing on genetic abnormality can make informed decision as to alternative family plans, such as adoption. • Prenatal testing can also indicate if the embryo or fetus is carrying genetic abnormalities. • Amniocentesis, Chorionic Villus Sampling, Ultrasound, and Blood Tests are examples of prenatal testing.

36. Amniocentesis • Performed on mother usually around 14 – 16 weeks after conception, although sometimes earlier • Amniotic Fluid containing cells sloughed off by fetus is withdrawn from the amniotic sac with a syringe. • Cells are separated, grown in a culture, and examined for chromosomal abnormalities. • Routine test for women over 35 to detect for Down’s Syndrome as chances for syndrome dramatically increase as women approach age 40. • Test also can determine the sex of child. • Amniocentesis does carry some risk of miscarriage. • Approx. 1 in 100

37. Figure 2.6 – Amniocentesis

38. Chorionic Villus Sampling (CVS) • Similar to Amniocentesis • Performed between 9 - 12 weeks • Syringe inserted through vagina into uterus and sucks out threadlike projections (villi) from the outer membrane that covers the amniotic sac and fetus. • Results available in days • CVS slightly higher risk than amniocentesis of spontaneous abortion; both increase the risk of miscarriage

39. Ultrasound • Sound waves that are too high in frequency to be heard by human ear are used to obtain information about the fetus. • Ultrasound waves are reflected by the fetus, and the computer uses the information to generate a picture of the fetus. • Picture is termed a sonogram. • Used to guide the syringe in amniocentesis and CVS and to locate fetal structures when intrauterine transfusions are needed for survival of fetus as with Rh disease • Used to track growth of fetus, detect multiple pregnancies, detect structural abnormalities • Procedure is able to detect sex of fetus

40. Blood Tests • Used to identify sickle-cell anemia, Tay-Sachs disease, and cystic fibrosis • Alpha-fetoprotein (AFP) assay used to detect neural tube defects such as spina bifida and chromosomal abnormalities. • Neural tube defects cause elevation in the AFP level in the mother’s blood. • High AFP levels related to increased risk of fetal death.

41. LO5 Heredity and the Environment © Fancy/Veer/Corbis

42. Heredity and the Environment • Inheritance, nutrition, learning, exercise, accident, and illness all influence development of traits. • Genotypes • Set of traits we inherit from our parents (our blueprint) • Phenotypes • Actual set of traits; develop because of both genetic and environmental influences (what actually transpires) • Canalization • Environmental conditions may prevent an individual from reaching their full potential, but if environmental influences improve, there is a tendency to “snap back” to the genetically determined “canal.” • Personality and intelligence are apparently less canalized (environment playing stronger role than with physical development).

43. Genetic - Environmental Correlation • One problem in sorting out the influences of heredity (nature) and environment (nurture) is that genes partly determine the environments to which people are exposed. • Psychologist Sandra Scarr describes 3 types of correlations between genetic and environmental influences as they relate to age of individuals: • Passive Correlation • Parents intentionally and unintentionally place children in certain environments; it is called passive because child has no choice • Evocative Correlation • A child’s genotype is connected with behaviors that elicit or evoke certain types of social responses from others, and these responses in turn become part of the environment of the child • Active Correlation • As we mature, we become more proactive in creating our environment. • Choosing environments that allow us to develop inherited preferences is termed niche-picking.

44. The Epigentic Framework • The relationship between genetic and environmental influences is not a one-way street; it is BIDIRECTIONAL. • Genes affect the development of traits and behaviors, but likewise traits and behaviors lead us to certain environments. • According to Epigenesis, development is a continuum of bidirectional exchanges between nature and nurture. © Brand X Images/Jupiterimages

45. Strategies for Understanding • Research strategies used to aid in sorting out effects of heredity and environment • Kinship Studies • Study distribution of traits among relatives with differing degrees of relatedness • If genes are dominant in manifesting a trait, people who are most closely related should be more likely to share it. • Twin Studies • Study both MZ twins sharing 100% genes and DZ twins sharing 50% genes • Study of MZ twins raised apart from infancy show similarities much the same as those raised together, leading us to believe there is a strong genetic component to traits. • Adoption Studies • Study children separated from biological parents at an early age • When children raised by adoptive parents demonstrate more similarities of traits with natural parents, again a strong argument for genetic predominance seems to prevail.

46. LO6 Conception: Against All Odds © Fancy/Veer/Corbis

47. CONCEPTION: The Players (Ova & Sperm) • Conception: union of an ovum and a sperm cell that occurs when the chromosomes of each combine to form 23 new pairs • Ova • Women are born with approx. 400,000 ova - all they will ever have but in an immature form • During woman’s reproductive years, approx. 400 (1 in 1,000) will ripen and be released. • Ova are much larger than sperm cells but barely visible to the human eye. • Sperm • Develop in stages, starting with 46 chromosomes, and after meiosis ends with 23 - half X’s and half Y’s • Each is about 1/500th of an inch - one of the smallest types of cells in the body.

48. CONCEPTION: The Journey & Meeting Against All Odds • Journey of the OVUM • At signal from female hormones (estrogen and progesterone), some ova begin to mature (puberty). • Approx. once a month one, midway through menstrual cycle, one (maybe more) is released from follicle into nearby fallopian tube • Takes 3 to 4 days to reach the uterus • Not self-propelled; need assistance from cilia (small hair-like structures) and contractions of tube wall • If not fertilized, it is discharged through uterus and vagina along with endometrium formed to support an embryo in the menstrual flow. © iStockphoto.com

49. Figure 2.8 – Female Reproductive Organs

50. CONCEPTION: The Journey & Meeting Against All Odds • Journey of the SPERM • Y chromosomes have greater motility (faster swimmers). • 120 to 150 boys are conceived for every 100 girls. • Male fetuses experience higher rate of spontaneous abortion during first month of pregnancy. • At birth, boys outnumber girls by ratio of 106:100. • Boys also have higher incidence of infant mortality, further equalizing sex ratio.