1. A lethal genotype causes death before reproduction can occur. This affects expected Mendelian ratios because non-survivors cannot reproduce.
2. In complete dominance, one allele is expressed and one allele is not. In incomplete dominance, the heterozygous phenotype is intermediate between those of the homozygotes. In codominance, both alleles are expressed.
3. Epistasis occurs when one gene influences the phenotype of another. This happens between different genes, not on the same gene.
5. A heterozygote with mild sickle cell anemia is an example of codominance, since both the normal and sickle cell allele is expressed.
6. Omar provides an example of epistasis, since the dominant gene for black hair Bb was affected and not expressed.
7. In penetrance, either all individuals express a gene or all individuals do not. Expressivity deals with the different level of expression.
8. Pleiotropy refers to a gene that can have multiple functions. This condition can make it difficult to accurately determine particular disorders.
9. Genetic heterogeneity refers to mutations in different genes that show the same phenotype. This can affect inheritance patterns because it can cause misunderstandings, since someone who is homozygous recessive for a mutation would not cause the condition.
10. Phenocopy is a trait caused by the environment, which appears to be an inherited trait.
11. The inheritance pattern of mitochondrial DNA differs from inheritance of single-gene traits because mitochondrial DNA is passed only from mother to child, since sperm does not include mitochondria. Single-gene traits can be inherited by mother or father.
12. Mitochondrial DNA is more prone to mutation than DNA in the nucleus because it has fewer types of DNA repair.
13. Heteroplasmy is a condition in which mitochondria in the same cell have different alleles of a particular gene.
14. Linked genes do not assort independently, and refer to the inheritance of genes on the same chromosome. Linked genes give a phenotypic ratio of 3:1, whereas unlinked genes assort independently with the expected phenotypic ratio of 9:3:3:1.
15. Crossover frequency increases when genes are at opposite ends since there is more space to cross. Genes that are closer together are less likely to cross over.
16. The first human linkage maps were constructed by R.P. Donohue while looking at chromosomes in white blood cells, and found chromosome variations linked to family traits.
17. Linkage disequilibrium occurs when DNA sequences are inherited together, and not completely random.
18. An LOD score represents the likelihood of crossover linkage, as opposed to chance encounters.
19. The sex chromosome composition of males is XY; for females it is XX.
20. Until week 5, male and female development are similar. At week 5, embryos develop unspecialized gonads. If Mullerian ducts develop, the embryo becomes a female. If Wolffian ducts persist, the embryo becomes a male.
21. The X chromosome has 1,500 genes, while the Y chromosome has just 231. The structure of the Y chromosome is unusual because it does not cross over along its length, and presents a palindrome type symmetry in its DNA.
22. Some conditions that disrupt sexual development are: androgen insensitivity syndrome, in which a person looks female, but is in fact male because of a mutation on the X chromosome resulting in the absence of androgen receptors. Intersex is another, in which an individual has internal structures different than their external structures, or who have genitalia which are considered ambiguous.
23. Homosexuality is considered to reflect input from both genes and the environment. Homosexuality has been tied to strong inclinations reflected in young children, pointing to a genetic component. This phenomena has also been linked to identical twins.
24. Sex ratio is the proportion of males to females in a population, calculated as males/females x 1000. Sex ratios favor females because they live longer.
25. Males have a single X chromosome, but females require two copies, as in autosomal traits.
26. X-linked recessive traits are more commonly expressed in males because a man can receive this trait from the mother since he only has one X chromosome, as opposed to two X chromosomes with females.
28. Sex-limited traits affect a structure/function of one particular sex. Sex-influenced traits have a dominant allele in one particular sex while recessive in the other sex.
29. X inactivation compensates for differences between males and females by balancing the gene expression on the X chromosome. Since females have two alleles and males have one, 75% of the genes on an X chromosome are turned off.
30. X inactivation begins when the embryo has 8 cells.
31. X inactivation is noticeable in heterozygotes because an allele is either expressed on one X chromosome or the other.
32. Genomic imprinting occurs when methyl groups prevent protein synthesis by covering a single gene or several linked genes.
33. Abnormal imprinting can cause disease when a normally imprinted allele is expressed while the other is not.
34. Differences in the timetables of sperm and oocyte formation can cause parent-of-origin effects because males can miscopy genes on 250 million sperm, versus 400,000 eggs in women.
35. Polygenic traits concern multiple genes, while Mendelian traits concern a single gene.
36. Multifactorial traits concern influences from the environment.
37. “Continuously varying” refers to polygenic traits that are created by a combination of several genes. A bell curve describes the distribution of phenotype classes of any polygenic trait because it demonstrates a categorization of individuals into classes. When these frequencies are calculated and plotted, we see a bell curve.
38. The fingerprint pattern is a multifactorial trait because it analyzes unique patterns from genes and the environment, which compares unique ridges, like a fingerprint.
39. Height is a multifactorial trait because it illustrates the impact of both genes and the environment, such as nutrition.
40. Skin is a multifactorial trait because skin color genes can also assort due to factors in genes and environment. Adaptations to sunny environments can influence this.
41. Empiric risk is based on the frequency in a population, not genetically influenced.
42. Heritability focuses on the genetic component of variation. If completely dependent of genetic action, heritability will equal 1.0.
43. Adopted individual studies assume that similarities with the parents are a result of environmental influences, while those of adoptees and their biological parents are more genetic. With twins, concordance is studied, particularly as it relates to the percentage of pairs that both twins express the trait among twins where one has at least one of the traits.
44. The twins that display 55% concordance for anorexia is likely monozygotic, and the twins displaying 7% concordance for anorexia is likely dizygotic.
45. Genome-wide association studies compare genetic markers between two populations of people, investigating one population with a particular trait and one without. Identification can help researchers determine which genes contribute to a phenotype.
46. Leptin decreases appetite, which should lower weight. Ghrelin on the other hand increases appetite and may cause a gain in weight. These two protein hormones counteract each other.
47. Obesity has a heritability of 75%, while BMI is lower. This leads to the assertion that genes play a large role in weight gain. It is therefore easier to gain weight than lose it.
48. Populations that become sedentary suddenly and switch to a high-calorie diet reveals that environment is a significant factor in further weight gain. The “thrifty gene hypothesis” posits that genes enabled Pima Indians to store fat. However, when a sedentary, high calorie environment was introduced, these people retained too much fat.
49. Behavior genetics is involved with the nervous system, including the characterstics of mood and mind.
50. The cellular basis of behavior is based on 100 billion nerve cells which make up our human brain. Genes influence behavior by controlling the neurotransmitters which facilitate communication.
51. Twin studies, particularly in monozygotic twins, showed significant commonalities in sleep duration, schedule, quality, and nap habits. This study showed how genes can influence sleep qualities. One mutation that alters sleep is narcolepsy, in which the subject suddenly falls asleep throughout the day. Narcolepsy is caused by a mutation in a gene that encodes a receptor called hypocretin. A second mutation is seen in advanced sleep phase syndrome, which affects the sleep-wake cycle. A mutation in a gene on chromosome 2 affects this cycle.
52. Intelligence is generally defined as the ability to reason, learn, remember, connect ideas, deduce, and create. It is measured based on an “intelligence quotient,” or IQ test.
53. The general intelligence value, or “g”, represents verbal fluency, mathematics reasoning, memory, and spatial visualization skills. This improves upon using IQ scores because it incorporates genetic and environmental factors.
54. Technologies such as exome sequencing are used to compare the genes of close relatives in order to detect mutations that may cause intellectual disabilities.
55. Drug addiction is the act of compulsive drug activity even when the dangers are known. Heritability has been indicated on twin studies, which point to gene variants and proteins that interfere with brain function.
56. Humans become addicted to a chemical from plants when these substances bind to receptors on neurons. This indicates that our bodies have versions of these chemicals that are naturally used by the body.
57. Nicotine addiction is caused by the release of dopamine, which fades away in minutes. Smoking to obtain nicotine must continue to extend this pleasurable release. The nicotinic receptor normally binds acetylcholine, but the nicotine molecule is similar enough to bind. Acetylcholine stimulates dopamine release.
58. The symptoms of major depressive disorder include difficulty falling and staying asleep, challenges with routine tasks, and weight loss.
59. Depression can be attributed to genetic and environmental influences equally. Studies indicate that genes involved in the control of circadian rhythms show greater instances of depression. Genes affecting the deficiency of serotonin can also affect the risk of depression.
60. Bipolar disorder symptoms include mania, which causes a person to become hyperactive and restless, with a rush of ideas and excitement.
61. In a bipolar disorder complex, many gene variant combinations are at play, suggesting that hundreds of genes may be responsible.
62. Schizophrenia symptoms include loss of the ability to organize thoughts and perceptions, delusions and hallucinations, and inappropriate reactions to news.
63. Genes play a strong role in susceptibility to schizophrenia, with a heritability of 0.8. Environmental factors however are also considered. Cytokines for example may alter brain development. Living in cities, migration, marijuana use, and childhood victimization may also be factors.
64. Autism is an impairment in socialization and communication skills.
65. Autism heritability is 90 percent. Evidence suggests single-gene disorders and chromosomal conditions may be linked to autism. Environmental triggers point to prenatal exposures to rubella and the drug valproate, in addition to exposure to folic acid supplements late in gestation.
66. Changing genetic technology has led to the discovery of genes implicated in autism with exome and whole-genome sequencing projects that allowed for the identification of many genes behind the disease. Technologies allow enormous datasets to be analyzed to isolate autism genes.