Chapter 23: Patterns of Gene Inheritance

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 (ex: big noses) that cause generations to resemble each other.

Gregor Mendel Mendel’s law of segregation: 1.) Each individual has two factors (called genes) for each trait (one from each parent). 2.) The genes segregate (separate) during gamete formation (i.e., meiosis). 3.) Each gamete contains only one gene for each trait (i.e., they are haploid). 4.) Fertilization gives the new individual two genes for each trait (one from each parent, restores diploid state).

Diploid = Two copies of each type of chromosome Loci = Physical position of a gene on a chromosome Homologous Chromosomes Genes From Father From Mother Allele = Alternate forms of a gene: Alleles have the same position (locus) on a pair of homologous chromosomes

Alleles code for the same trait. Examples of alleles: curly or straight (alleles), hair type (gene) attached or unattached (alleles), ear lobe type (gene) Chromosomes segregate during the formation of the gametes and each gamete has only one chromosome from each pair. Fertilization gives each new individual two chromosomes again.

The Inheritance and Expression 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 only in the absence of a dominant allele. An example is w for a continuous or straight hairline.

STRAIGHT or CONTINUOUS HAIRLINE Widow’s peak WIDOW’S PEAK WW Ww In humans, widow’s peak (top) is dominant over straight hairline (bottom). STRAIGHT or CONTINUOUS HAIRLINE ww

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.

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 – widow’s peak or straight hairline. Both WW and Ww result in widow’s peak, two genotypes with the same phenotype.

Gamete Formation Because homologous pairs separate during meiosis, a gamete has only one allele from each pair of alleles (for a specific gene). If the allelic pair is Ww, the resulting gametes would contain either a W or a w, but not both – (gametes are haploid). Ww represents the genotype of an individual. Gametes that could be produced by this individual are W or w.

Genotype AA Aa AABb AaBb Gametes A A A a AB Ab AB Ab aB ab

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 monohybrids when they are heterozygous for one pair of alleles.

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

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. W w WW Ww ww 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.)

Genotypes of parents are known (both are heterozygous Ww) Monohybrid cross Genotypes of parents are known (both are heterozygous Ww) Genotypic Ratio WW homozygous dominant Ww heterzygous ww homozygous recessive Phenotypic Ratio widow’s peak 1 straight hairline 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.

The One-Trait Testcross It is not always possible to discern a homozygous dominant from a heterozygous individual by inspection of phenotype (they have the same phenotype – both will have widow’s peak). A testcross crosses the dominant phenotype with the recessive phenotype. If a homozygous recessive phenotype is among the offspring, the parent must be heterozygous.

One-trait testcross ? All offspring have dominant phenotype. Therefore the dominant parent (genotype we are tying to figure out) must be homozygous dominant. 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.

? Offspring have dominant and recessive phenotypes. Therefore the dominant parent (genotype we are tying to figure out) must be heterozygous dominant. Because the offspring show a 1:1 phenotypic ratio, the individual is heterozygous as shown.

1.) Both a man and woman are heterozygous for tongue rolling. Tongue rolling is dominant over non-tongue rolling. What is the chance that their child will be a tongue roller? Male Female Tt GENOTYPE Tt T t GAMETES T t T t FEMALE TT Tt T t Offspring Phenotypes 3 Rollers Non-Roller 3 of 4 chances for roller child (75% chance). MALE Tt tt

2) Both you and your sibling are non-rollers and your parents are rollers. Tongue rolling is dominant over non-tongue rolling. What are the genotypes of your parents? OFFSPRING tt PARENTS T t T t T t TT Tt Tt tt

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.

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.

Two-Trait Crosses In two-trait crosses, genotypes of the parents require four letters because there two alleles for each trait. Gametes will contain one letter for each trait. When a dihybrid (heterozygous for both traits) reproduces with another dihybrid the phenotypic results are 9 : 3 : 3 : 1.

Widow’s Peak is dominant over Straight Hairline W w Short Fingers are dominant over Long Fingers S s Phenotype Genotypes Widow’s Peak / Short Fingers WWSS WWSs WwSS WsSs Widow’s Peak / Long Fingers WWss Wwss Straight HL / Short Fingers wwSS wwSs Straight HL / Long Fingers wwss

Dihybrid cross (two traits) Widow’s Peak Short Fingers Straight Hairline Long Fingers homozygous dominant homozygous recessive 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 Widow’s Peak Short Fingers

WwSs WwSs

The Two-Trait Testcross A testcross is done to determine genotype of individual that has dominant phenotypes (for both traits). (Homozygous dominant or heterozygous for the two traits under consideration). Cross heterozygote for both traits with homozygous recessive for both traits - results in 1 : 1 : 1 : 1 ratio.

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.

SELECTED TRAITS IN HUMAN HEREDITY Dominant Recessive normal skin pigmentation albinism freckles no freckles broad lips thin lips tongue roller non-tongue roller PTC taster PTC non-taster large eyes small eyes migraine headaches no migraine headaches normal foot arch flat feet

If a man that is homozygous recessive for eye size (i.e., has small eyes) and is homozygous dominant for freckles (i.e., has freckles) has children with a woman that is homozygous dominant for eye size (i.e., has large eyes) and is homozygous recessive for freckles (i.e., does not have freckles), what are the potential phenotypes and genotypes of their children? Man llFF  IF only for gametes Woman LLff  Lf only for gametes GENOTYPE PHENOTYPE IF IF All are heterozygous for both traits and show large eyes with freckles Lf ILFf ILFf Lf ILFf ILFf

If one of the children reproduces with another person that has the same genotype, what are the chances that they will have a child with large eyes and freckles? LlFf X LlFf LF Lf lF lf large eyes/freckles 9/16 or  56 % LF LLFF LLFf LlFF LlFf Large eyes/no freckles Lf 3/16 LLFf LLff LlFf Llff Small eyes/freckles lF LlFF LlFf llFF llFf 3/16 Small eyes/no freckles lf LlFf Llff llFf llff 1/16

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. Genetic counselors construct pedigree charts to determine the mode (dominant or recessive) of inheritance of a condition.

Pedigree Analysis: determine how a genetic disorder is inherited, chances of offspring having a genetic disorder. Unaffected male Unaffected female Affected Male Affected female “UNION” OFFSPRING

Genetic Disorders: medical conditions caused by alleles inherited from parents, hereditary disorder. Autosomal Genetic Disorders: genetic disorders caused by Alleles on autosomal chromosomes (non-sex Chromosomes – similar to somatic). Autosomal Disorders can be: 1) Autosomal Dominant 2) Autosomal Recessive Autosomal Dominant AA or Aa have disorder (phenotype) aa Aa Aa aa

Autosomal Recessive: aa have disorder (phenotype) AA or Aa aa aa aa aa Aa

Autosomal recessive pedigree chart (Tay-sachs disease, Cystic fibrosis, PKU) CARRIER – Has allele but is unaffected * * HOW DO YOU KNOW INDIVIDUAL IS HETEROZYGOUS? 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?

Autosomal dominant pedigree chart (Neurofibromatosis, Huntington disease) * * HOW DO YOU KNOW INDIVIDUAL IS HETEROZYGOUS? 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?

Polygenic Inheritance Polygenic traits are governed by more than one gene pair (e.g., several pairs of genes may be involved in determining the phenotype).

Polygenic inheritance Such traits produce a continuous variation representing a bell-shaped curve (Ex: height in humans). 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.

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 from very dark to very light. The distribution of these phenotypes also follows a bell-shaped curve.

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.

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 (A, B, and O). ***Each individual inherits only two alleles for these genes.

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). Alleles: A, B, O 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.

Inheritance of blood type…. (Who’s your daddy?) A mating between blood type A and blood type B can result in any one of four blood types.

Incompletely Dominant Traits Codominance means that both alleles are equally expressed in a heterozygote. (Ex: sickle cell anemia) Incomplete dominance is exhibited when the heterozygote doesn’t show the dominant trait but shows an intermediate phenotype, representing a blending of traits. (Ex: curly, wavy, or straight hair) The alleles A and B in ABO blood groupings are codominant. An example of incomplete dominance is seen in degree of hair curliness.

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.

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.

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.