1. Chapter 23: Patterns of Gene Inheritance

2. Mendel’s Laws
Gregor Mendel was an Austrian monk who in 1860 developed certain laws of heredity after doing crosses between garden pea plants.
Gregor Mendel investigated genetics at the organismal level.
Examples of traits that can be observed at the organismal level include facial features that cause generations to resemble each other.

3. Mendel working in his garden
Mendel grew and tended the pea plants he used for his experiments. For each experiment, he observed as many offspring as possible. For a cross that required him to count the number of round seeds to wrinkled seeds, he observed and counted a total of 7,324 peas.

4. Gregor Mendel
Gregor Mendel combined his farmer’s skills with his training in mathematics.
Mendel’s law of segregation states that each individual has two factors (called genes today) for each trait.
Alternative forms of a gene having the same position on a pair of homologous chromosomes and affecting the same trait are now referred to as alleles.

5. Today we know that alleles occur at the same loci (position) on a chromosome.
The factors segregate during the formation of the gametes and each gamete has only one factor from each pair.
Fertilization gives each new individual two factors again.

6. Gene locus
Each allelic pair, such as Gg or Zz, is located on homologous chromosomes at a particular gene locus, shown at the left. Following replication, each sister chromatid carries the same alleles in the same order, as shown at the right.

7. The Inheritance of a Single Trait
A capital letter indicates a dominant allele, which is expressed when present.
An example is W for widow’s peak.
A lowercase letter indicates a recessive allele, which is only expressed in the absence of a dominant allele.
An example is w for continuous hairline.

8. Widow’s peak
In humans, widow’s peak (top) is dominant over straight hairline (bottom).

9. Genotype and Phenotype
Genotype refers to the genes of an individual which can be represented by two letters or by a short descriptive phrase.
Homozygous means that both alleles are the same; for example, WW stands for homozygous dominant and ww stands for homozygous recessive.

10. Heterozygous means that the members of the allelic pair are different—for example, Ww.
Phenotype refers to the physical or observable characteristics of the individual.
Both WW and Ww result in widow’s peak, two genotypes with the same phenotype.

11. Gamete Formation
Because homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles.
If the allelic pair is Ww, a gamete would contain either a W or a w, but not both.
Ww represents the genotype of an individual.
Gametes are represented by W or w.

12. One-Trait Crosses
In one-trait crosses, only one trait such as type of hairline is being considered.
When performing crosses, the original parents are called the parental generation, or the P generation.
All of their children are the filial generation, or F generation.
Children are monohydrids when they are heterozygous for one pair of alleles.

13. P stands for the parental generation; F for the filial generation
P stands for the parental generation; F for the filial generation. The F generation from this cross are all monohybrids.

14. When a monohybrid reproduces with a monohybrid, the results are 3 : 1.
If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring.
When a monohybrid reproduces with a monohybrid, the results are 3 : 1.
This ratio is used to state the chances of a particular phenotype.
A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.
The 3:1 phenotypic ratio indicates three offspring with the dominant phenotype and 1 with the recessive phenotype. (The genotypic ratio would be 1:2:1.)

15. Monohybrid cross
A Punnett square diagrams the results of a cross. When the parents are heterozygous, each child has a 75% chance of having the dominant phenotype and a 25% chance of having the recessive phenotype.
It is important to realize that chance has no memory; for example, if two heterozygous parents already have three children with a widow’s peak and are expecting a fourth child, this child still has a 75% chance of having a widow’s peak and a 25% chance of having a straight hairline.

16. One-Trait Crosses and Probability
Laws of probability alone can be used to determine results of a cross.
The laws are:
(1) the probability that two or more independent events will occur together is the product of their chances occurring separately, and
(2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.

17. In the cross of Ww x Ww, what is the chance of obtaining either a W or a w from a parent?
Chance of W = ½, or chance of w = ½
The probability of these genotypes is:
The chance of WW = ½ x ½ = ¼
The chance of Ww = ½ x ½ = ¼
The chance of wW = ½ x ½ = ¼
The chance of ww = ½ x ½ = ¼
The chance of widow’s peak (WW, Ww, wW) is ¼ + ¼ + ¼ = ¾ or 75%.

18. The One-Trait Testcross
It is not always possible to discern a homozygous dominant from a heterozygous individual by inspection of phenotype.
A testcross crosses the dominant phenotype with the recessive phenotype.
If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.

19. One-trait testcross
A testcross determines if an individual with the dominant phenotype is homozygous or heterozygous. Because all offspring show the dominant characteristic, the individual is most likely homozygous as shown.

20. Because the offspring show a 1:1 phenotypic ratio, the individual is heterozygous as shown.

21. The Inheritance of Many Traits
Independent Assortment
The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the gametes.
This law is dependent on the random arrangement of homologous pairs at metaphase.

22. Segregation and independent assortment
Segregation occurs because the homologous chromosomes separate during meiosis I. Also, independent assortment occurs. The homologous chromosomes line up randomly at the metaphase plate; therefore, the homologous chromosomes, and the alleles they carry, segregate independently during gamete formation. All possible combinations of chromosomes and alleles occur in the gametes.

23. Two-Trait Crosses
In two-trait crosses, genotypes of the parents require four letters because there is an allelic pair for each trait.
Gametes will contain one letter of each kind in every possible combination.
When a dihybrid reproduces with a dihybrid the results are 9 : 3 : 3 : 1.

24. Dihybrid cross
F1 is the first filial generation, offspring of the parental cross. F2, or second filial generation, is the offspring of the cross of two F1 individuals.
Since each F1 parent can form four possible types of gametes, four different phenotypes occur among the offspring in the proportions shown.
The expected F2 phenotypic ratio is:
9 widow’s peak and short fingers
3 window’s peak and long fingers
3 straight hairline and short fingers
1 straight hairline and long fingers

25. Two-Trait Crosses and Probability
It is possible to use the two laws of probability to arrive at a phenotypic ratio for a two-trait cross without using a Punnett square.
The results for two separate monohybrid crosses are as follows:
Probability of widow’s peak = ¾
Probability of short fingers = ¾
Probability of straight hairline = ¼
Probability of long fingers = ¼

26. The probabilities for the dihybrid cross:
Probability of widow’s peak and short fingers = ¾ x ¾ = 9/16
Probability of widow’s peak and long fingers = ¾ x ¼ = 3/16
Probability of straight hairline and short fingers = ¼ x ¾ = 3/16
Probability of straight hairline and long fingers = ¼ x ¼ = 1/16

27. The Two-Trait Testcross
A testcross is done when it is not known whether a dihybrid individual is homozygous dominant or heterozygous for both or one of the traits under consideration.
A cross of a person heterozygous for both traits with a homozygous recessive person produces a 1 : 1 : 1 : 1 ratio.

28. Two-trait testcross
A testcross determines if the individual with a dominant phenotype is homozygous or heterozygous. If the individual is heterozygous as shown, there is a 25% chance for each possible genotype.

29. Patterns of Inheritance
Genetic Disorders
Patterns of Inheritance
When studying human disorders, biologists often construct pedigree charts to show the pattern of inheritance of a characteristic within a family.
The particular pattern indicates the manner in which a characteristic is inherited.

30. Pedigree charts represent males as squares and females as circles.
Recessive and dominant alleles have different patterns of inheritance.
Genetic counselors construct pedigree charts to determine the mode of inheritance of a condition.

31. Autosomal recessive pedigree chart
Autosomal recessive disorders have these characteristics:
Affected children can have unaffected parents.
Heterozygotes (Aa) have a normal phenotype.
Two affected parents will always have affected children.
Affected individuals with homozygous dominant mates will have unaffected children.
Close unaffected relatives who reproduce are more likely to have affected children if they have joint affected relatives.
Both males ad females are affected with equal frequency.
How would you know the individual at the asterisk is heterozygous?

32. Autosomal dominant pedigree chart
Autosomal dominant disorders have these characteristics:
Affected children will have at least one affected parent.
Heterozygotes (Aa) are affected.
Two affected parents can produce an unaffected child.
Two unaffected parents will not have affected children.
Both males and females are affected with equal frequency.
How would you know the individual at the asterisk is heterozygous?

33. Autosomal Recessive Disorders
Tay-Sachs Disease
Tay-Sachs disease is common among United States Jews of central and eastern European descent.
An affected infant develops neurological impairments and dies by the age of three or four.
Tay-Sachs results from a lack of hexosaminidase A and the storage of its substrate in lysosomes.
Prenatal diagnosis of Tay-Sachs is possible following

34. Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disorder among Caucasians.
A chloride ion transport protein is defective in affected individuals.
Normally when chloride ion passes through a membrane, water follows.
In cystic fibrosis patients, a reduction in water results in a thick mucus which accumulates in bronchial passageways and pancreatic ducts.
Genetic testing for the CF gene is possible.

35. Cystic fibrosis therapy
Antibiotic therapy is used to control lung infections of cystic fibrosis patients. The antibiotic tobramycin can be aerosolized and administered using a nebulizer. It is inhaled twice daily for about fifteen minutes.

36. Phenylketonuria (PKU)
Individuals with phenylketonuria lack an enzyme needed for the normal metabolism of phenylalanine, coded by an allele on chromosome 12.
Newborns are regularly tested for elevated phenylalanine in the urine.
If the infant is not put on a phenylalanine-restrictive diet in infancy until age seven when the brain is fully developed, brain damage and severe mental retardation result.

37. Autosomal Dominant Disorders
Neurofibromatosis
Small benign tumors, made up largely of nerve cells, occur under skin or on various organs.
The effects can range from mild to severe, and some neurological impairment is possible; this disorder is variably expressive.
The gene for this trait is on chromosome 17.
Neurofibromatosis is variably expressive; most individuals have mild symptoms, but some have severe symptoms.

38. Huntington Disease
Individuals with Huntington disease experience progressive degeneration of the nervous system and no treatment is presently known.
Most patients appear normal until middle age.
The gene coding for the protein huntingtin contains many more repeats of glutamines than normal.
The gene for Huntington disease is located on chromosome 4.

39. Huntington disease
Persons with this condition gradually lose psychomotor control of the body. At first, the disturbances are only minor, but the symptoms become worse with time.

40. Beyond Simple Inheritance Patterns
Polygenic Inheritance
Polygenic traits are governed by more than one gene pair.
Several pairs of genes may be involved in determining the phenotype.
Such traits produce a continuous variation representing a bell-shaped curve.

41. Polygenic inheritance
When you record the heights of a large group of young men, the values follow a bell-shaped curve. Such a continuous distribution is due to control of a trait by several sets of alleles. Environmental effects (i.e., differences in nutrition) are also involved.

42. Skin Color
The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance.
A range of phenotypes exist and several possible phenotypes fall between the two extremes of very dark and very light.
The distribution of these phenotypes follows a bell-shaped curve.

43. Polygenic Disorders
Many human traits, like allergies, schizophrenia, hypertension, diabetes, cancers, and cleft lip, appear to be due to the combined action of many genes plus environmental influences.
Many behaviors, such as phobias, are also likely due to the combination of genes and the effects of the environment.

44. Multiple Allelic Traits
Inheritance by multiple alleles occurs when more than two alternative alleles exist for a particular gene locus.
A person’s blood type is an example of a trait determined by multiple alleles.
Each individual inherits only two alleles for these genes.

45. ABO Blood Types
A person can have an allele for an A antigen (blood type A) or a B antigen (blood type B), both A and B antigens (blood type AB), or no antigen (blood type O) on the red blood cells.
Human blood types can be type A (IAIA or IA i), type B (IBIB or IBi), type AB (IAIB), or type 0 (ii).
The Rh factor is inherited separately from ABO blood types. When you are Rh positive, your red blood cells have a particular antigen, and when you are Rh negative, that antigen is absent. The Rh-positive allele is dominant over the Rh-negative allele.

46. Inheritance of blood type
A mating between blood type A and blood type B can result in any one of four blood types.

47. Incompletely Dominant Traits
Codominance means that both alleles are equally expressed in a heterozygote.
Incomplete dominance is exhibited when the heterozygote shows not the dominant trait but an intermediate phenotype, representing a blending of traits.
Such a cross would produce a phenotypic ratio of 1 : 2 : 1.
The alleles A and B in ABO blood groupings are codominant. An example of incomplete dominance is seen in degree of hair curliness.

48. Incomplete dominance
Among Caucasians, neither straight nor curly hair is dominant. When two wavy-haired individuals reproduce, each offspring has a 25% chance of having either straight or curly hair and a 50% chance of having wavy hair, the intermediate phenotype.

49. Sickle-Cell Disease
Sickle-cell disease is an example of a human disorder controlled by incompletely dominant alleles.
Sickle cell disease involves irregular, sickle shaped red blood cells caused by abnormal hemoglobin.
HbA represents normal hemoglobin; and HbS represents the sickled condition.
Individuals with sickle cell trait survive malaria because the sickling of some red blood cells causes a leakage of potassium, which is toxic to the malarial parasite.

50. HbAHbA individuals are normal; HbSHbS individuals have sickle-cell disease and HbAHbS individuals have the intermediate condition called sickle-cell trait.
Heterozygotes have an advantage in malaria-infested Africa because the pathogen for malaria cannot exist in their blood cells.
This evolutionary selection accounts for the prevalence of the allele among African Americans.

51. Chapter Summary
Alleles are alternative forms of a gene located at one site on a chromosome; alleles determine the traits of individuals.
Chromosomes and their alleles separate and assort independently when gametes form; this increases variety among offspring.

52. Many genetic disorders and other traits are inherited according to laws first established by Gregor Mendel.
Inheritance is often more complex, providing exceptions to Mendel’s laws but helping to explain an even wider variety in patterns of gene inheritance.