copyright © 2013 pearson education, inc. all rights reserved. chapter 4 genetics: from genotype to...

Copyright © 2013 Pearson Education, Inc. All rights reserved.

Chapter 4Genetics: From Genotype to

Phenotype


From Genotype to Phenotype

Structural Genes Regulatory Genes


From Genotype to Phenotype (cont’d)

Genotype Phenotype


From Genotype to Phenotype (cont’d)

• The ABO Blood Type System– Recessive, dominant and co-dominant alleles

• Obesity: A Complex Interaction– Genes, environment, and phenotype


Mendelian Genetics

• Gregor Mendel (1822-1884)• Experiments in particulate inheritance• Discovered dichotomous variation• Developed a series of postulates about

inheritance

Gregor MendelA monk living in

what is now the Czech Republic

Crossed different strains of purebred plants and studied their progeny.

His work illustrates the basic rules of inheritance.


Mendel’s Postulates

• Hereditary characteristics are controlled by particulate unit factors that exist in pairs in individual organisms

• When an individual has two different unit factors responsible for a characteristic, only one is expressed and is said to be dominant to the other, which is said to be recessive


Mendel’s Postulates (cont’d)

• Mendel’s Law of Segregation: during the formation of gametes, the paired unit factors separate or segregate randomly so that each sex cell receives one or the other with equal likelihood

• Mendel’s Law of Independent Assortment: during gamete formation, segregating pairs of unit factors assort independently of each other

HybridsOffspring of parents that differ from

each other with regard to certain traits or certain aspects of genetic makeup; heterozygotes.

Traits Mendel Studied in Peas

Principle of SegregationDiscrete units, or genes, occur in pairs

because chromosomes occur in pairs.

During gamete production, members of each gene pair separate so each gamete contains one member of a pair.

During fertilization, the full number of chromosomes is restored and members of a gene or allele pairs are reunited.

Results When One Trait (Height) Is Considered at a

Time

DominanceDominant traits are governed by an allele

that can be expressed in the presence of another, different allele. Dominant alleles prevent the

expression of recessive alleles in heterozygotes.

RecessivenessRecessive traits are not expressed

in heterozygotes.For a recessive allele to be

expressed, there must be two copies of the allele.

AllelesAlternate forms of a gene.

Alleles occur at the same locus on a pair of chromosomes and influence the same trait.

However, because they are slightly different, their action may result in different expressions of that trait.

The term is sometimes used synonymously with gene.

LocusThe position on a chromosome

where a given gene occurs.

The term is sometimes used interchangeably with gene, but this usage is technically incorrect.

PhenotypesThe observable or detectable

physical characteristics of an organism; the detectable expressions of genotypes, frequently influenced by environment.

Principle of Independent Assortment

The distribution of one pair of alleles into gametes does not influence the distribution of another pair.

The genes controlling different traits are inherited independently of one another.

Random AssortmentThe chance distribution of

chromosomes to daughter cells during meiosis; along with recombination, a source of genetic variation (but not new alleles) from meiosis.

Mendelian TraitsCharacteristics that are influenced

by alleles at only one genetic locus.

Examples include many blood types, such as ABO.

Many genetic disorders, including sickle- cell anemia and Tay-Sachs disease, are also Mendelian traits.

Mendelian Inheritance in Humans

Over 19,000 human traits are known to be inherited according to Mendelian principles (Online Mendelian Inheritance in Man www.ncbi.nlm.nih.gov/omim/)

The human ABO blood system is an example of a simple Mendelian inheritance.The A and B alleles are dominant to the O

allele.Neither the A or B allele are dominant to one

another; they are codominant and both traits are expressed.

CodominanceThe expression of two alleles in

heterozygotes.

In this situation, neither allele is dominant or recessive; thus, both influence the phenotype.

What Dominance Doesn’t Mean

Complete determinant of phenotype

Better or stronger

Polygenic InheritancePolygenic traits, or continuous

traits, are governed by alleles at two or more loci, and each locus has some influence on the phenotype.

Hair, eye and skin color are polygenic traits


Linkage and Assortment

• Linkage: genes found on the same chromosome are said to be linked. The closer together two genes are, the greater the linkage

• Crossing Over: makes possible the independent assortment of linked genes


X-Linked Disorders

Genetic conditions that result from mutations to genes

on the X chromosome. They are almost always

expressed in males, who have only one copy

of the X chromosome.


Mendelian Genetics in Humans

• Over the past century, hundreds of human disorders and diseases have been cataloged, which can be explained in terms of Mendelian genetic transmission– Earlobe form– Phenylthiocarbamide (PTC) tasting or non-

tasting


Mendelian Genetics in Humans (cont’d)

• The Online Mendelian Inheritance in Man (OMIM) Web site: (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM ) provides an extraordinary database on genetic conditions in humans, from the most innocuous to the most lethal


Genetics Beyond Mendel

• Qualitative variation• Phenotypic variation that can be characterized as

belonging to discrete, observable categories

• Quantitative variation• Phenotypic variation that is characterized by the

distribution of continuous variation within a population


Genetics Beyond Mendel (cont’d)

• Polygenic Traits, the Phenotype, and the Environment– Heritability

• The proportion of total phenotypic variability observed for a given trait that can be ascribed to genetic factors



Variability caused by genetics

Variability caused by genetics + Variability caused by the environment



• Heritability and IQ Test Score Performance– IQ Test Score Performance

• Exhibit continuous variation in human populations, with a normal distribution

• Genetics is an important factor in producing the variation observed within populations

• Both genetics and environment effect IQ score variation


Phenylketonuria: Illustrating Mendelian and Post-Mendelian Concepts

• Autosomal recessive condition that leads to the accumulation of large quantities of the amino acid phenylalanine, which causes mental retardation and other phenotypic abnormalities

– Screening in newborns– Nutritional prognosis


Genes and Environments

• Environment– From a gene’s perspective the “environment” is

made up of other genes• The genetic environment is just as critical to the

production of phenotypes as any other kind of environment

copyright © 2013 pearson education, inc. all rights reserved. chapter 4 genetics: from genotype to...

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different traits

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different allele

allele pairs

certain traits

pair of chromosomes

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