BIOLOGY OF HUMANS Concepts, Applications, and Issues Fifth Edition Judith Goodenough Betty McGuire 20 Genetics and Human Inheritance Lecture Presentation Anne Gasc Hawaii Pacific University and University of Hawaii Honolulu Community College
Genetics and Human Inheritance OUTLINE: Principles of Inheritance Breaks in Chromosomes Detecting Genetic Disorders
Principles of Inheritance Genetic information is carried on chromosomes that are in the egg and sperm in equal numbers Homologous pairs of chromosomes 23 chromosomes received from one parent pair with 23 chromosomes from the other parent Each member of a homologous pair carries genes for the same traits Genes Segments of DNA Code for a specific protein that will play a structural or functional role in the cell
Principles of Inheritance Trait Characteristic Produced by the actions of one or more gene-directed proteins
Principles of Inheritance Alleles Different forms of a gene Produce different versions of the trait they determine Example: gene for freckles One allele causes freckles to form Other allele does not
Figure 20.1 Important terms in genetics.
Principles of Inheritance Homozygous (homo, same; zygo, joined together) Individuals with two copies of the same allele Heterozygous (hetero, different) Individuals with different alleles of a given gene Dominant (use upper case example A ) When the effects of an allele can be detected regardless of the alternative allele Recessive (use lower case example a ) When the effects of an allele are masked in the heterozygous condition
Principles of Inheritance Genotype Alleles that are present Genetic composition of an individual Phenotype Observable physical traits of an individual For example, the freckled phenotype has two genotypes: FF and Ff
Figure 20.2 Genotypes for selected human phenotypes.
Table 20.1 Review of Common Terms in Genetics
Gamete Formation Law of segregation During gamete formation, the two alleles for each gene separate as the homologous chromosomes move toward opposite ends of the cell during meiosis Each chromosome is inherited independent of the other chromosomes, following the law of independent assortment
Figure 20.3 Gamete formation.
Mendelian Genetics Gregor Mendel was a monk in the nineteenth century who grew up in a region of what was then Austria and is now part of the Czech Republic Studied how single genes are inherited from parent to offspring Used pea plants First used one-trait crosses Then used two-trait (dihybrid) crosses
Mendelian Genetics Punnett square Matrix used to predict genetic makeup of offspring of individuals of particular genotypes Rows represent possible gametes of one parent Columns represent possible gametes of the other parent Boxes represent possible combinations of gametes
Figure 20.4 Punnett square.
One- and Two-Trait Crosses Monohybrid cross Cross in which both parents are heterozygous for one trait of interest Genotypic ratio of offspring: 1 FF : 2 Ff : 1 ff Phenotypic ratio of offspring: 3 with freckles (FF and Ff) : 1 without (ff) Dihybrid cross Cross in which both parents are heterozygous for two traits of interest Phenotypic ratio of offspring: 9 : 3 : 3 : 1
One- and Two-Trait Crosses Web Activity: One- and Two-Trait Crosses
Figure 20.5 (a) Gamete formation (b) Punnett square.
Figure 20.6 A person who is heterozygous gametes.
Figure 20.7 A dihybrid cross.
Pedigrees Chart showing the genetic connections among individuals in a family Especially useful in following recessive alleles that are not visible in the heterozygote
Figure 20.8 Pedigrees.
Pedigrees Genetic disorders Often caused by recessive alleles Carrier Someone who displays the dominant phenotype but is heterozygous for a trait Carries the recessive allele and can pass it to descendants
Dominant and Recessive Alleles Dominant allele Often produces a functional protein that the recessive allele does not Example: albinism Lacking the ability to produce brown pigment melanin Ability to produce melanin depends on the enzyme tyrosinase Dominant allele that results in pigmentation produces functional tyrosinase Recessive allele that results in albinism produces nonfunctional tyrosinase
Codominant Alleles Complete dominance Heterozygote exhibits the trait associated with the dominant allele but not that of the recessive allele Codominance Effects of both alleles are apparent in a heterozygote Example: blood type AB The protein products of both the A and B alleles are expressed on the surface of the red blood cell
Incomplete Dominance Incomplete dominance Expression of the trait in a heterozygous individual is in between the way the trait is expressed in a homozygous dominant or homozygous recessive person Example: sickle-cell allele Heterozygote has sickle-cell trait (Hb A Hb S )
Incomplete Dominance Web Activity: Codominance and Incomplete Dominance
Figure 20.11 The inheritance of sickle-cell trait.
Pleiotropy One gene having many effects Besides providing an example of incomplete dominance, sickle-cell anemia is an example of pleiotropy Sickling of red blood cells caused by abnormal hemoglobin affects many areas of the body The sickled cells can break down, clog blood vessels, and accumulate in the spleen These effects can affect the heart, brain, lungs, kidneys, and muscles and joints
Figure 20.12 Sickle-cell anemia.
Multiple Alleles When three or more forms of a given gene exist across many people in the population Example: ABO blood types Blood type is determined by the presence of certain polysaccharides (sugars) on the surface of red blood cells Type A blood has the A polysaccharide Type B has the B polysaccharide Type AB has both A and B polysaccharides Type O has neither
Multiple Alleles When three or more forms of a given gene exist across many people in the population (cont d) Gene has three alleles: I A, I B, I Alleles I A and I B specify the A and B polysaccharides, respectively When both of these alleles are present, both polysaccharides are produced I A and I B are, therefore, codominant
Table 20.2 The Relationship between Genotype and ABO Blood Types
Polygenic Inheritance Variation in a trait, such as height, independent of environmental influences Involves two or more genes, often on different chromosomes Many traits, including height, skin color, and eye color, vary almost continuously from one extreme to another Environment can play a role in creating such a smooth continuum
Figure 20.13 Human height varies along a continuum.
Genes on the Same Chromosome Usually inherited together Described as being linked Linked genes usually do not assort independently Usually is emphasized here because there is a mechanism that can unlink genes on the same chromosome: crossing over
Sex-Linked Genes Y is much smaller than X and carries fewer genes Most genes on the X chromosome have no corresponding alleles on the Y chromosome Known as X-linked genes Different pattern of inheritance: recessive phenotype of X-linked genes more common in males because son can inherit X-linked recessive only from mother Examples of disorders Red-green color blindness Two forms of hemophilia Duchenne muscular dystrophy
Sex-Linked Genes Web Activity: Sex-Linked Traits
Figure 20.14 Genes that are X linked.
Sex-Influenced Genes Autosomal genes whose expression is influenced by sex hormones Their expression differs in males and females Example: male pattern baldness 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 Men can be heterozygous for the trait and still show pattern baldness Women who are homozygous for the trait will develop pattern baldness later in life
Breaks in Chromosomes Usually caused by chemicals, radiations, viruses Results in changes in the structure and function of the chromosome Deletion Loss of a piece of chromosome Most common deletion occurs when the tip of a chromosome breaks off Example: cri-du-chat syndrome Loss of tip of chromosome 5
Figure 20.15 Cri-du-chat syndrome.
Breaks in Chromosomes Duplication Addition of piece of chromosome Effects depend on size and position of the addition Example: Fragile X syndrome Duplication of a region on the X chromosome
Figure 20.16 Fragile X syndrome.
Detecting Genetic Disorders Prenatal genetic testing is recommended when A defective gene runs in the family The mother is older than 35, due to increased risks of nondisjunction
Detecting Genetic Disorders Amniocentesis 10 to 20 ml of amniotic fluid is withdrawn, which contains epithelial cells of the fetus Cells are cultured and then examined Abnormalities in the number of chromosomes Presence of certain alleles that are likely to cause specific diseases
Detecting Genetic Disorders Chorionic villi sampling (CVS) Involves taking a small piece of chorionic villi Fingerlike projections of the chorion Cells of chorion have same genetic makeup as fetus Cells are cultured and then chromosomes examined A disadvantage of CVS is that it has a slightly greater risk of triggering miscarriage than does amniocentesis
Figure 20.17 Amniocentesis and chorionic villi sampling.
Genetic Testing Newborn genetic testing Blood test screens for phenylketonuria (PKU) Allows doctors and parents to prevent brain damage by keeping the infant on a strict diet that excludes most phenylalanine Adult genetic testing Many predictive genetic tests are now available or being developed Some identify people who are at risk or predisposed for a specific disease before symptoms appear
You Should Now Be Able To: Know the principles of inheritance and the definitions for gene, allele, dominant, recessive, phenotype, genotype, homozygous, heterozygous Understand the Mendelian genetic: Mendel, Punnett square, monohybrid and dihybrid crosses Understand codominance, incomplete dominance, polygenic inheritance, pleiotropy, X-linked gene, and sex-influenced trait Know the breaks in chromosomes Understand the detection of genetic disorders