Chapter Seventeen- Simple Patterns of Inheritance

Gregor Mendel

(1822-1884) entered monastery and became a priest. historic experiments with pea plants. his paper was ignored at the time, but his findings were independently rediscovered years later

Procedure for cross-fertilization

1. Pollen from one plant is placed on the stigma of a flower from a different plant 2. This is accomplished by prying open an immature flower and removing the stamens 3. Then take the pollen from another flower's stamens and place on the stigma 4. Using this procedure, Mendel was able to make controlled crosses

Mendel's three important ideas

1. traits are dominant and recessive 2. genes and alleles 3. segregation of alleles

Testcross

A dwarf pea plant must be tt. A tall pea plant could be either TT or Tt, so genotype must be determined by a testcross. Cross the unknown individual (TT or Tt) to a homozygous recessive individual (tt) If some offspring are dwarf, unknown individual must have been Tt If all offspring are tall, unknown individual was TT

Pedigree for a dominant trait

A family pedigree for Huntington disease, an autosomal dominant trait.

continuous variation: inheritance pattern

A pattern in which the offspring display a continuous range of phenotypes

epistasis: inheritance pattern

A type of gene interaction in which the alleles of one gene mask the effects of an allele of another gene.

Mendel's Law of Independent Assortment

Alleles of different genes assort independently of each other during gamete formation.

Epistasis

Alleles of one gene mask the expression of the alleles of another gene • Often arise because two or more different proteins are involved in a single cellular function • Example: In sweet peas, a colorless precursor molecule must be acted on by two different enzymes to produce purple pigment

Sweet pea flower pigment

Cross between true-breeding purple flowers and true-breeding white produced (as expected): F1 - all purple-flowered plants F2 - 3:1 purple to white-flowered • Cross between two varieties of white-flowered peas crossed produced surprising results: F1 - all purple-flowered plants F2 - 9:7 purple to white-flowered • Two genes are involved in flower color C (purple) dominant to c (white) P (purple) dominant to p (white) If either cc or pp homozygous, flowers are white

Types of traits

Discrete Clearly defined phenotypic variants Purple or white flowers, red or white eyes • Quantitative Majority of traits Continuous variation over a range of phenotypes Example: height, skin color, number of fruits on a tree Polygenic - multiple genes contribute to phenotype Environment also plays a role

Traits are dominant and recessive

Dominant variant is displayed in hybrids Recessive variant is masked by dominant

At meiosis, one member of each chromosome pair segregates into each daughter nucleus.

During the formation of haploid cells, the members of different chromosome pairs segregate independently of each other. Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.

incomplete dominance: molecular basis

Fifty percent of the protein encoded by the functional (wild-type) allele results in an intermediate phenotype.

Two-factor cross

Follows inheritance of two different traits • Can determine linkage • Possible patterns: Two genes are linked - variants found together in parents are always inherited as a unit Two genes are independent - variants are randomly distributed Dihybrid offspring - offspring are hybrids with respect to both traits Data for F2 hybrids is consistent with independent assortment

Chromosomes contain the genetic material (DNA).

Genes are found in the chromosomes.

Incomplete dominance

Heterozygote shows intermediate phenotype Neither allele is dominant

x-linked inheritance: molecular basis

In a female with one recessive X-linked allele (a heterozygote), the protein encoded by the dominant allele is sufficient to produce the dominant trait. A male with a recessive X-linked allele does not have a dominant allele and does not make any of the functional protein.

X-linked traits

In humans, X chromosome is larger and carries more genes than the Y chromosome Genes found on the X but not the Y are X-linked genes • Sex-linked genes are found on one sex chromosome but not the other Males are hemizygous for X-linked genes • Example: Hemophilia A Hemophilia A is caused by recessive X-linked gene • Disease allele h A X encodes defective version of a clotting protein

Pedigree Analysis of Human Traits

Inherited trait is analyzed over the course of several generations in one family

Disease genes can be recessive or dominant, autosomal or sex-linked

Many of the alleles causing human genetic disease are recessive, like Cystic Fibrosis But some are dominant, like Huntington disease • Huntington disease has an autosomal dominant inheritance pattern • Gene is on one of 22 pairs of autosomes Disease genes can also be found on the sex chromosomes

The nucleus of a diploid cell contains two sets of chromosomes, found in homologous pairs.

Maternal and paternal sets of homologous chromosomes are functionally equivalent; each set carries a full complement of genes

Chromosomes and segregation

Mendel's Law of Segregation can be explained by the pairing and segregation of homologous chromosomes during meiosis. The physical location of a gene on a chromosome is called its locus

Codominance

Multiple alleles - three or more variants in a population Phenotype depends on which two alleles are inherited Example: ABO blood types in humans Type AB is codominant - expresses both alleles equally

Different mechanisms of sex determination

Not all chromosomal mechanisms involve sex chromosomes Bees are haplo-diploid - male is haploid and female is diploid • Other mechanisms also exist Sex is controlled by environment (temperature) in some reptiles and fish • Plants Some have a single type of plant making male and female gametophytes Others have sexually distinct plants making male or female gametophytes only

Genes and alleles

Particulate mechanism of inheritance His "unit factors" are genes Every individual has two genes for a character A gene has two variant forms, or alleles

x-linked inheritance: inheritance pattern

Pattern of traits is determined by genes that display a dominant/recessive relationship and are located on the X chromosome. In mammals and fruit flies, males are hemizygous for X-linked genes. In these species. X-linked recessive traits occur more frequently in males than in females.

codominance: inheritance pattern

Pattern that occurs when the heterozygote expresses both alleles simultaneously. For example, a human carrying the A and B alleles for the ABO antigens of red blood cells produces both the A and the B antigens (has an AB blood type).

incomplete dominance: inheritance pattern

Pattern that occurs when the heterozygote has a phenotype intermediate to the phenotypes of the homozygotes, as when a cross between red-flowered and white-flowered plants produces pink-flowered offspring.

Phenotype

Physical or behavioral characteristics that are the result of gene expression TT and Tt are tall tt is dwarf

Simple Mendelian Inheritance

Recessive allele does not affect phenotype of heterozygote • Single copy of the dominant allele makes enough functional protein to provide a normal phenotype, masking recessive allele • Sometimes heterozygote may even upregulate the lone functional allele to provide high enough expression

Sex Chromosomes and X-linked Inheritance Patterns

Sex chromosomes are found in many (but not all) species with two sexes Several mechanisms for sex determination

Variations in Inheritance Patterns and Their Molecular Basis

Simple Mendelian inheritance • Alleles are dominant or recessive • Phenotype ratios follow Mendel's laws More complex forms of inheritance • Incomplete dominance • Codominance Understanding gene function at the molecular level explains differences in inheritance patterns

Punnett square

Step 1. Write down genotypes of parents Male parent: Tt Female parent: Tt Step 2. Write down the possible gametes that each parent can make Male gametes: T or t Female gametes: T or t Step 3. Create an empty Punnett square Step 4. Fill in the possible genotypes. Step 5. Determine relative proportions of genotypes and phenotypes.

Chromosomes and independent assortment

The Law of Independent Assortment can also be explained by the behavior of chromosomes during meiosis. Random alignment of chromosome pairs during meiosis I leads to the independent assortment of genes found on different chromosome

codominance: molecular basis

The codominant alleles encode proteins that function somewhat differently from each other. In a heterozygote, the function of each protein affects the phenotype uniquely.

Genotype

The genetic composition of an individual TT - homozygous dominant tt - homozygous recessive Tt - heterozygous

Chromosomes are replicated and passed from parent to offspring.

They are also passed from cell to cell during the development of a multicellular organism.

continuous variation: molecular basis

This pattern is produced by the additive interactions of several genes, along with environmental influences.

Segregation of alleles

Two copies of a gene carried by an F1 plant segregate (separate) from each other, so that each sperm or egg carries only one allele F2 traits follow approximately 3:1 ratio

Mendel's Law of Segregation

Two copies of a gene segregate from each other during the transmission from parent to offspring.

epistasis: molecular basis

Two different genes are needed to produce a given phenotype. Loss of function of one of the genes alters the phenotype.

single-factor cross

Where the experimenter follows only a single trait P generation: True-breeding parents F1 generation: Offspring of P cross Monohybrids (if parents differ in one trait) F2 generation: F1 self-fertilizes Recessive trait reappears

X-O system

females XX and males X or XO

simple mendelian inheritance: molecular basis

in many cases, the recessive allele is nonfunctional. though a heterozygote may produce 50% of the functional protein compared with a dominant homozygote, this is sufficient to produce the dominant trait.

X-Y system

males XY and females XX

Z-W system

males ZZ and females ZW

simple mendelian inheritance: inheritance pattern

pattern of traits is determined by a pair of alleles that display a dominant/recessive relationship and are located on an autosome. The presence of the dominant allele masks the presence of the recessive allele.

Garden Pea, Pisum sativum

several advantageous properties: many different variable traits, normally self-fertilizing- Female gamete fertilized by male gamete from same plant Easy to breed true-breeding lines (exhibit the same trait). large flowers make crosses easy when desired- Cross-fertilization or hybridization