The Principles of Inheritance Chapter 20 The Principles of Inheritance

Genes Code for Proteins Humans have about 22,000 genes; most code for a protein. Proteins have specific functions leading to specific traits (i.e., phenotypes) Protein functions include: hormones, enzymes, receptors, structural, neurotransmitters, etc. Mutation: change in nucleotide sequence of DNA  change in AA sequence of protein.

Background Review and Definitions Each human cell has 46 (23 pairs) of chromosomes Homologous chromosomes: one maternal, one paternal

FIGURE 20.1 Important terms in genetics

End of Quiz 2

Principles of Inheritance The actions of one or more proteins results in a trait, or inherited characteristic

Principles of Inheritance Each copy of the pair is often slightly different and are called alleles

Principles of Inheritance Individuals who inherited the same allele from each parent are homozygous Those with different alleles of the same gene are heterozygous

Dominant Alleles When the effects of an allele are expressed to the exclusion of the alternative allele, then that allele is described as dominant

Recessive Alleles If the effects of an allele are masked in the heterozygous condition, then the allele is described as recessive

Background Review and Definitions Genotype: complete set of genes and alleles Phenotype: observed physical and functional traits Examples: eye color, hair color, height, blood group, etc.

TABLE 20.1 REVIEW OF COMMON TERMS IN GENETICS

Alleles Segregate During Meiosis The law of segregation says that during gamete formation: any two alleles on homologous chromosomes separate as the chromosomes move toward opposite ends of the cell during meiosis

Alleles Assort Independently Each chromosome is inherited independent of the other chromosomes following the law of independent assortment

The law of independent assortment states that chromosomes: Cross over Are inherited independently Stick together when they assort Condense

Segregation of Genes During Meiosis FIGURE 20.3 Gamete formation by a female who is homozygous dominant for freckles (FF), a male who is homozygous recessive for no freckles (ff), and the heterozygous (Ff) individual resulting from the union of these gametes.

Segregation of Genes During Meiosis FIGURE 20.5a (a) Gamete formation by a person who is heterozygous for the freckle trait (Ff).

Which of the following are inherited (i. e Which of the following are inherited (i.e., segregated) independently during meiosis? Cytoplasm Mitochondria Ribosomes Chromosomes Alleles

Which of the following are inherited (i. e Which of the following are inherited (i.e., segregated) independently during meiosis? Cytoplasm Mitochondria Ribosomes Chromosomes Alleles

Punnett Square A Punnett square is a matrix where the rows represent the possible gametes of one parent, the columns the possible gametes of another parent, and the boxes the possible combinations of gametes

Both parents homozygous Punnett Square FIGURE 20.4 This Punnett square illustrates the probable offspring from a cross between a homozygous dominant female with freckles (FF) and a homozygous recessive male without freckles (ff). The columns are labeled with the possible gametes the male could produce (one gamete per column). The rows are labeled with the possible gametes the female could produce (one gamete per row). Combining the labels on the corresponding rows and columns yields the genotype of possible offspring. Punnett Square

Both parents heterozygous FIGURE 20.5b (b) A Punnett square showing the probable outcome of a mating between two people who are heterozygous for the freckle trait (Ff).

Punnett Square Analysis

Principles of Inheritance A pedigree is a chart showing the genetic connections among individuals in a family

FIGURE 20.8a Pedigrees showing the inheritance of (a) a dominant autosomal trait and (b) a recessive autosomal trait. A pedigree is constructed so that each generation occupies a different horizontal line, numbered from top to bottom, with the most ancestral at the top. Males are indicated as squares, and females as circles. A horizontal line connects the partners in a marriage. An affected individual is indicated with a solidly colored symbol.

I II FIGURE 20.8b Pedigrees showing the inheritance of (a) a dominant autosomal trait and (b) a recessive autosomal trait. A pedigree is constructed so that each generation occupies a different horizontal line, numbered from top to bottom, with the most ancestral at the top. Males are indicated as squares, and females as circles. A horizontal line connects the partners in a marriage. An affected individual is indicated with a solidly colored symbol. III

What is the genotype of the male in the second (II) generation? cc Cc CC Could be either CC or Cc FIGURE 20.8b Pedigrees showing the inheritance of (a) a dominant autosomal trait and (b) a recessive autosomal trait. A pedigree is constructed so that each generation occupies a different horizontal line, numbered from top to bottom, with the most ancestral at the top. Males are indicated as squares, and females as circles. A horizontal line connects the partners in a marriage. An affected individual is indicated with a solidly colored symbol. III

What are the genotypes of the males in the third (III) generation? cc Cc CC Could be either CC or Cc FIGURE 20.8b Pedigrees showing the inheritance of (a) a dominant autosomal trait and (b) a recessive autosomal trait. A pedigree is constructed so that each generation occupies a different horizontal line, numbered from top to bottom, with the most ancestral at the top. Males are indicated as squares, and females as circles. A horizontal line connects the partners in a marriage. An affected individual is indicated with a solidly colored symbol. III

Principles of Inheritance Genetic disorders are often caused by recessive alleles Someone who displays the dominant phenotype but is heterozygous for a trait is a carrier of the recessive allele

Principles of Inheritance A dominant allele often produces a protein that the recessive allele does not This is the case in albinism where the ability to produce the brown pigment melanin is lacking

FIGURE 20.10 A person with albinism lacks the brown pigment melanin in the skin, hair, and irises of the eyes.

Complete Dominance In the previous examples, complete dominance was the situation where the heterozygote exhibited the trait associated with the dominant allele but not that of the recessive allele

Codominance Codominance is the case when the effects of both alleles are apparent to their fullest extent in a heterozygote

Codominance This is the case in the blood type AB where the protein products of both the A and B alleles are expressed on the surface of the red blood cell

? FIGURE 20.11 The inheritance of ABO blood type is an example of codominance. The alleles IA and IB are codominant; they are both expressed. Both these alleles are dominant over i.

TABLE 20.2 THE RELATIONSHIP BETWEEN GENOTYPE AND ABO BLOOD GROUPS

Incomplete Dominance Incomplete dominance is the expression of the trait in a heterozygous individual that is in between the way the trait is expressed in the homozygous dominant or homozygous recessive person

Incomplete Dominance The sickle-cell allele shows incomplete dominance. The heterozygote is described as having sickle-cell trait (HbAHbS)

Sickle Cells

FIGURE 20.12 The inheritance of sickle cell trait (HAHS) is an example of incomplete dominance.

Principles of Inheritance Sometimes one gene can have many effects The effect of the sickle hemoglobin affects many areas of the body

FIGURE 20.13 Sickle cell anemia is an example of pleiotropy, a condition in which a single gene has many effects.

Codominance Only one allele is expressed Neither allele is expressed Both alleles are fully expressed One allele is dominant and the other is recessive

Multiple Alleles When three or more forms of a given gene exist across many people in the population, they are referred to as multiple alleles

Multiple Alleles The ABO blood group has three alleles, IA, IB, and IO. However, any one person can only have two of these alleles

TABLE 20.2 THE RELATIONSHIP BETWEEN GENOTYPE AND ABO BLOOD GROUPS

Polygenic Inheritance Variation among a trait such as height, independent of environmental influences, results from polygenic inheritance or the involvement of two or more genes in the determination of the trait

FIGURE 20.14a Human height varies along a continuum. (a) One reason is that height is determined by more than one gene (polygenic inheritance). This figure shows the distribution of alleles for tallness in children of two parents of medium height, assuming that three genes are involved in the determination of height. The top line of boxes shows the parental genotypes, and the second line of boxes indicates the possible genotypes of the offspring. Alleles for tallness are indicated with dark squares. (b) Students organized according to height.

Sex-Linked Genes Sex-linked genes are located on the X chromosome X chromosome: larger contains many genes Y chromosome: smaller contains very few genes

X-Linked Genes Thus most genes on the X chromosome have no corresponding alleles on the Y chromosome and are known as X-linked genes

FIGURE 20.15 Genes that are X linked have a different pattern of inheritance than do genes on autosomes, as seen in this cross between a carrier mother and a father who is normal for the trait. The recessive allele is indicated in red. Notice that each son has a 50% chance of displaying the recessive phenotype. All daughters will appear normal, but each daughter has a 50% chance of being a carrier.

Sex-Linked Inheritance: X and Y Chromosomes Y determines male sex SRY gene Sex-linked (or X-linked) inheritance Characteristics: mostly males affected. Passed to sons by mother Father cannot pass the gene to son Common disorders include: Color blindness, hemophilia, Duchenne muscular dystrophy, Androgen insensitivity syndrome

Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease

What are the chances that a couple will have a color-blind son if the woman is heterozygous for the X-linked color-blind gene and the man has normal color vision? 100% 75% 50% 25% 0%

40 chromosomes 20 chromosome 10 chromosomes 5 chromosomes Suppose that the normal diploid content of a cell (before meiosis begins) is 20 chromosomes (10 pairs). How many chromosomes will be present in each gamete at the end of meiosis? 40 chromosomes 20 chromosome 10 chromosomes 5 chromosomes 0 chromosomes

Sex Influenced Genes Sex-influenced genes are autosomal genes whose expression is influenced by sex hormones (e.g., testosterone or estrogen)

Sex Influenced Genes Male pattern baldness is more common in men than in women because its expression depends on both the presence of the allele for baldness and the presence of testosterone

Breaks in Chromosomes Change Structure and Function Chromosome breakage results in changes in the structure and function of the chromosome

Chromosome Deletion The loss of a piece of chromosome is called a deletion The most common deletion is when the tip of a chromosome breaks off

FIGURE 20.16 Occurring in 1 out of every 50,000 live births, cri-du-chat syndrome is the most common genetic deletion found in humans. It is caused by the loss of a small region near the tip of chromosome 5. The name of the syndrome, which means cry of the cat, describes the sound of the cry of affected babies. The infants have a characteristically round face and wide-set eyes.

Chromosome Duplication An added piece of chromosome is called a duplication The effects of a duplication depend on its size and position

Breaks in Chromosomes Change Structure and Function Genetic disorders also occur when certain sequences of DNA nucleotides are duplicated multiple times as in fragile X syndrome

FIGURE 20.17 Fragile X syndrome. (a) Duplication of a region on the X chromosome makes the chromosome fragile and easily broken. (b) A child with fragile X syndrome appears normal. (c) Characteristics of an adult with fragile X syndrome include a long face and large ears. In addition, fragile X syndrome causes mental retardation.

Certain Genetic Disorders Can Be Detected Before Birth by Amniocentesis or Chorionic Villi Sampling

Amniocentesis Performed at 12-20 weeks of pregnancy In amniocentesis 10-20 ml of amniotic fluid are withdrawn which contain epithelial cells of the fetus

Amniocentesis

Amniocentesis These cells are cultured and then examined for abnormalities in the number of chromosomes and the presence of certain alleles that are likely to cause specific diseases

FIGURE 20.18 part 2 Amniocentesis and chorionic villi sampling (CVS) are procedures available for prenatal genetic testing.

Chorionic Villi Sampling Chorionic villi sampling (CVS) involves taking a small piece of chorionic villi, fingerlike projections of the chorion The tissue sample is then examined for abnormalities Can be performed several weeks earlier than amniocentesis

FIGURE 20.18 part 1 Amniocentesis and chorionic villi sampling (CVS) are procedures available for prenatal genetic testing.

Certain Genetic Disorders Can Be Detected by Laboratory Tests Many predictive genetic tests are now available or are being developed Some of these tests identify people who are at risk or predisposed for a specific disease before symptoms appear