10
(p. 188)
10.1 Mendel and the Garden Pea (p. 188; Figs. 10.1, 10.2, 10.3)
10.2
10.3
A.
Heredity is the passing along of traits from one generation to the next, and genetics is the study of heredity.
B.
A scientist and monk by the name of Gregor Mendel contributed to the understanding of heredity in the 1800s by being the first to actually count numbers of offspring in crosses involving pea plants.
C.
Early Ideas About Heredity
1.
Over 200 years before Mendel’s work, British farmers had tried similar crosses and noted that some types had a stronger tendency to pass along their traits.
D.
Mendel’s Experiments
1.
Gregor Mendel was born in 1822 and educated in a monastery.
2.
He became a monk and later joined a science club for which he undertook a number of studies.
E.
Mendel’s Experimental System: The Garden Pea
1.
Mendel used the garden pea because many varieties were available, previous work had been done with these plants, peas are small and grow quickly, and left alone, the flowers self-pollinate.
F.
Mendel’s Experimental Design
1.
Mendel began his crosses with true-breeding varieties that contained only one version of a trait.
2.
The first generation in a succession of crosses is the P or parental generation; their offspring are the F
1
generation or first filial generation.
3.
Offspring of two members of the F
1
generation comprise the F
2
generation.
What Mendel Observed (p. 190; Figs. 10.4, 10.5; Table 10.1)
A.
contrasting traits.
B.
The F
1
Generation
1.
Mendel called the trait expressed in the F
1
plants the dominant trait, and the trait not expressed was recessive.
C.
The F
2
Generation
1.
When the F
1
plants were allowed to self-fertilize, Mendel found 3:1 dominant to recessive phenotype in the F
2
generation.
2.
Mendel’s studies were unique because he actually counted the number of offspring with each trait.
D.
A Disguised 1:2:1 Ratio
1.
When F
2
plants were allowed to self-fertilize, Mendel found a 1:2:1 ratio of truebreeding dominant to not true-breeding dominant to true-breeding recessive.
Mendel Proposes a Theory (p. 192; Figs. 10.6, 10.7, 10.8, 10.9; Table 10.2)
A.
Based on his results, Mendel proposed a set of hypotheses that comprised a theory of heredity.
1. Information about traits is transmitted in genes.
2. Individuals have two copies of the gene for each trait. When both copies are the same, the individual is said to be homozygous for that trait. If the two genes are different, the individual is heterozygous.
3. Alternate forms of the gene for each trait are called alleles. Phenotype refers to the outward expression of the genes. The genetic makeup of an individual, what alleles an individual has, is the genotype.
4. The two alleles do not affect each other, and recessive alleles may not be expressed.

10.4
B.
Analyzing Mendel’s Results
1.
Genetics traits are usually represented by a letter symbol for convention. Dominant alleles are assigned an uppercase letter, and recessive alleles are assigned the same letter, but lowercase.
C.
Punnett Squares
1.
A modified multiplication table, called a Punnett square, is an easy way to organize all possible genotypes when conducting a genetic cross.
2.
The likelihood of getting one particular genotype in the offspring is expressed as a probability.
D.
The Testcross
1.
When Mendel did not know the genotype of an individual expressing a dominant trait, he did a testcross by crossing the individual with a homozygous recessive for the trait.
2.
Testcrosses can also be used to determine the genotype of an individual when two genes are involved.
Mendel’s Laws (p. 195; Fig. 10.10)
A.
Mendel’s First Law: Segregation
1.
Mendel's First Law, or the Law of Segregation, says that only one of a pair of alleles is passed to a gamete.
B.
Mendel’s Second Law: Independent Assortment
1.
Mendel's Second Law was determined when he worked with two traits at a time in dihybrid crosses. This law, called the Law of Independent Assortment, states that genes located on different chromosomes are inherited independently.
(p. 196)
10.5 How Genes Influence Traits (p. 197; Fig. 10.11)
A.
From DNA to Protein
1.
Genes are encoded in DNA, and DNA is transcribed into RNA in the nucleus.
2.
RNA is translated into protein on ribosomes in the cytoplasm.
B.
How Proteins Determine the Phenotype
1.
The specific sequence of amino acids determines a protein’s functional shape, which affects how a protein will function, which in turn affects the phenotype.
C.
How Mutation Alters Phenotype
1.
A change in a single nucleotide within a gene can alter the amino acid encoded, thus changing the shape of a protein and affecting the phenotype.
D.
Natural Selection for Alternative Phenotypes Leads to Evolution
1.
Random mutations occurring in populations, although rare, may result in a better adaptation in a changing environment.
10.6 Some Traits Don’t Show Mendelian Inheritance (p. 198; Figs. 10.12–10.20)
A.
The expressions of genotype are not always straightforward.
B.
Continuous Variation
1.
Sometimes one trait, such as human height, is determined by the action of several genes, which results in a continuous variation for the trait within a population.
C.
Pleiotropic Effects
1.
When an allele affects more than one trait, it is said to be pleiotropic.
D.
Incomplete Dominance
1.
A condition known as incomplete dominance is seen when offspring exhibit a phenotype intermediate to that of both parents.
E.
Environmental Effects
1.
The degree to which an allele is expressed can sometimes depend on the environment.
2.
Some alleles are heat-sensitive, resulting in different pigmentation during seasonal weather changes.

F. Epistasis
1.
Epistasis is an interaction between the products of two genes in which one of the genes modifies the phenotypic expression of the other.
2.
In Emerson’s experiments on corn seed color, his modified ratios were the result of epistasis.
G. Codominance
1.
Sometimes more than two alleles, or multiple alleles, exist for a given trait in a population of individuals.
2.
An example is the human ABO blood designation. a.
In the human ABO blood group, there are three possible I gene alleles: I A , I B , and i.
b.
I A and I B alleles are codominant; the i allele is recessive. c.
Different combinations of the three alleles produce four different blood types (A,
B, AB, and O)
(p. 205)
10.7
10.8
Chromosomes Are the Vehicles of Mendelian Inheritance (p. 205; Figs. 10.21, 10.22, 10.23)
A.
The Chromosomal Theory of Inheritance
1.
Observations that similar chromosomes paired with each other during meiosis led to the chromosomal theory of inheritance.
2.
Other pieces of evidence that contributed were that reproduction involved the union of two cells, one egg cell and one sperm cell, and that diploid individuals have two copies of each heritable gene, and gametes each have one.
3.
Also, segregation and independent assortment in meiosis were observed, and were consistent with Mendel’s model.
B.
Problems with the Chromosomal Theory
1.
The observation that the number of traits that assorts independently in an organism exceeds the number of chromosome pairs the organism possesses made scientists wonder whether the chromosomal theory was correct.
C.
Morgan’s White-Eyed Fly
1.
Morgan’s 1910 studies of a white-eyed fruit fly showed that certain genes sometimes only showed up males.
D.
Sex Linkage Proves the Chromosomal Theory
1.
Male fruit flies have only one X chromosome, while female fruit flies have two X chromosomes. Genes that are carried on the sex chromosome are sex-linked.
2.
Eye color in this instance was sex-linked, which explained why males were white-eyed and females had red eyes.
3.
Morgan’s experiments were important because they illustrated that genes are carried on chromosomes and were consistent with Mendel’s laws.
4.
In linkage in general, the closer that genes are located to each other on a chromosome, the more likely they will segregate together, or be linked.
Human Chromosomes (p. 207; Figs. 10.24-10.28)
A.
Humans have 46 chromosomes (23 pairs) that vary by size, shape, and appearance.
B.
In humans, 22 pairs of chromosomes are perfectly matched and are called autosomes. The remaining pair of chromosomes is the sex chromosomes, two similar chromosomes in females (XX) and two dissimilar chromosomes in males (XY).
C.
Nondisjunction
1.
Sometimes during meiosis, the homologous chromosomes or the sister chromatids do not separate properly, a mistake known as nondisjunction.
2.
This leads to aneuploidy, which means having an abnormal number of chromosomes.
3.
Monosomics have lost one copy of an autosome and do not survive development.
Trisomics have gained an extra copy of an autosome (that is, they have three copies of a chromosome, rather than the normal two) and also usually do not survive, except in the cases of five of the smallest chromosomes.
4.
Down syndrome is an example of a trisomic condition in which the individual is born with an extra copy of chromosome 21.
5.
This condition results in mental impairment and a host of other physical defects.

D.
Nondisjunction Involving the Sex Chromosomes
1.
In humans, two kinds of sex chromosomes exist, an X chromosome and a Y chromosome. Human females have two X's, and males have an X and a Y.
2.
The Y chromosome is highly condensed and carries few functional genes; the genes that are active are responsible for features associated with “maleness.”
3.
Just as aneuploidy can occur with the autosomes, so it can also occur with the sex chromosomes. However, individuals who gain or lose a sex chromosome usually do survive, but with some abnormalities.
(p. 211)
10.9 Studying Pedigrees (p. 211; Figs. 10.29, 10.30)
A.
Analyzing a pedigree for albinism
B.
Analyzing a pedigree for color blindness
10.10 The Role of Mutations (p. 213; Figs. 10.31–10.35)
C.
Many human hereditary disorders reflect the presence of mutations within human populations.
D.
Family trees, or pedigrees, can indicate the mode of inheritance of the mutation.
E.
Hemophilia is a recessive, blood-clotting disorder. Some types of hemophilia are sex-linked.
F.
Sickle cell anemia is due to a recessive mutation of hemoglobin that is common in Africa because heterozygotes are resistant to malaria.
G.
Tay-Sachs Disease
1.
Tay-Sachs disease is an incurable hereditary disorder that progressively destroys the brain of those who are homozygous for the trait.
2.
It is carried as a recessive and is most common in Jews from Eastern and Central Europe.
H.
Huntington’s Disease
1.
Huntington’s disease is a dominant condition that does not express itself until later in life, after the trait has been passed on to the next generation.
10.11 Genetic Counseling and Therapy (p. 216; Figs. 10.36, 10.37)
A.
Genetic counseling can help couples predict the risk of bearing children with genetic defects.
B.
Genetic Screening
1.
Pregnant women who fear they may be at risk of carrying a fetus with a genetic defect can undergo a procedure called amniocentesis in which a minute amount of amniotic fluid surrounding the fetus is removed and checked for genetic defects.
2.
Ultrasound allows for viewing of the fetus without harming it.
3.
Chorionic villi sampling involves removal of a small portion of the chorionic villi for genetic testing.
 heredity (p. 188) Heredity is the passing of traits from parents to offspring.
 dominant (p. 190) Of a pair, the trait that is expressed.
 recessive (p. 190) Of a pair, the trait that is often not expressed.
 gene (p. 192)
 heterozygous (p. 192) Heterozygotes possess alternate forms for an allele; homozygotes have two copies of the same form of a gene.
 allele (p. 192)
 phenotype (p. 192) Phenotype refers to the outward expression of genes.
 genotype (p. 192) The alleles that an individual possess for a gene make up the genotype.

Punnett square (p. 193)
 testcross (p. 194)

Law of segregation (p. 195)

Law of Independent assortment (p. 195)
 pleiotropic (p. 198) When an allele affects more than one trait, it is said to be pleiotropic.

 incomplete dominance (p. 199)
 epistasis (p. 200) Epistasis occurs when the products of two genes interact to affect phenotype.
 codominant (p. 202) Human blood groups are an excellent example of codominance.
 sex-linked (p. 206) Some biologists prefer “X-linked” or “Y-linked” to avoid confusion, although most sex-linked traits are X-linked.
 autosome (p. 207)
 sex chromosomes (p. 207)
 nondisjunction (p. 208) Nondisjunction can occur either at meiosis I or II.
 aneuploidy (p. 208) Euploidy means the true or normal number of chromosomes; aneuploidy means more or less than the normal number.
 pedigree (p. 211)
 albinism (p. 207)
 color blindness (p. 208)
 hemophilia (p. 213)
 sickle-cell anemia (p. 214)

Tay-sachs disease (p. 215)

Huntington’s disease (p. 215)
 genetic counseling (p. 216)
 amniocentesis (p. 216)
 ultrasound (p. 216)
 chorionic villus sampling (p. 217)
 preimplantation genetic screening (p. 217)
1.
The study of genetics requires students to learn many new terms. When discussing the basic ideas of genetics, it is helpful to jot down the main terms with their definitions in simple language at the top or side edge of the chalkboard, and then leave them there during the rest of the discussion. These give students something to refer to and it helps them to learn concepts with less confusion. Similarly, it is helpful to do a number of genetics problems using Punnett squares, rather than doing just one or two examples.
2.
Genetics Problem— A Dihybrid. In Mendel's pea plants, purple flower color is dominant to white, and green pod color is dominant to yellow pod color. Have students cross a pea plant that is homozygous dominant for both traits with one that is homozygous recessive ( PPGG x ppgg ). All F
1
's will be heterozygous for both traits, and all will show both dominant phenotypes. Next, have students cross two F
1
's using a 4 x 4 Punnett square. They should come up with a phenotypic ratio of 9:3:3:1.
1.
You are a research scientist working in a genetics lab, and the institute that funds your research wants you to quickly determine the genetics of a newly discovered tropical plant that seems to have three flower color variations. The flowers have chemicals that could potentially be the source of life-saving medications. Describe the nature of the crosses you would make to determine a pattern of inheritance.
2.
Skin and hair color in humans exhibit a wide range in colors. After reading this chapter, what inheritance pattern do you believe is involved with color?
3.
Your sister has cystic fibrosis and you are contemplating having children of your own. Your mate recalls having a cousin with the disease. What is the risk that one or more of your children will have cystic fibrosis or is there any way to know for certain?