Introduction to Genetics

Chapter 19Human Biology, eighth edition

Cecie Starr et al

Impacts, Issues: The Color of Skin

Skin color comes from the pigment melanin• Produced by melanocytes in skin cells• More than 100 genes directly or indirectly

influence amount of melanin in an individual’s skin• Lead to many variations in skin color

Video: ABC News: All in the family: Mixed race twins

19.1 Basic Concepts of Heredity

Genes provide the instructions for all human traits, including physical features and how body parts function

Each person inherits a particular mix of maternal and paternal genes

Basic Concepts of Heredity (1)

Genes• Humans have ~21,500• Chemical instructions for building proteins• Locus: specific location on a chromosome

Diploid cells contain two copies of each gene on pairs of homologous chromosomes

Allele: each version of a gene

A Few Basic Genetic Terms

Many Genetic Traits Have Dominant and Recessive Forms

Basic Concepts of Heredity (2)

Homozygous condition: identical alleles

Heterozygous condition: different alleles

Dominant allele• Effect masks recessive allele paired with it

Basic Concepts of Heredity (3)

Genetic representations• Homozygous dominant (AA)• Homozygous recessive (aa)• Heterozygous (Aa)

Genotype• Inherited alleles

Phenotype • Observable functional or physical traits

Genotype and Phenotype Compared

19.2 One Chromosome, One Copy of a Gene

We inherit pairs of a genes (alleles) on pairs of chromosomes, but a gamete receives only one gene from each pair

One Chromosome, One Copy of a Gene

Monohybrid cross• Learn more about genotypes

Segregation• Pairs of alleles separated during gamete

formation

The Trait Called a Chin Fissure Arises from One Allele of a Gene

Animation: Chromosome segregation

Each Pair of Gene Alleles Is Separated and Two Alleles End Up in Different Gametes

19.3 Genetic Tools: Testcrosses and Probability

When potential parents are concerned about passing a harmful trait to a child, genetic counselors must try to predict the likely outcome of the mating

Probability

Measure of the chance that some particular outcome will occur

Factor in the inheritance of single-gene traits

Cross CC x cc• All of the offspring will be heterozygous, Cc

Cross Cc x Cc• ¼ CC, ½ Cc, and ¼ cc

A Punnett Square Can Be Used to Predict the Result of a Genetic Cross

Punnett square• Grid used to determine possible outcomes of

genetic crosses• Rules of probability apply because fertilization is

a chance event• Possibility can be expressed mathematically, e.g.,

between 0% and 100%

Most probable outcome does not have to occur

In a given situation, probability does not change

Making a Punnett Square Is One Way to Determine Likely Outcome of Genetic Cross

Different Genetic Results Possible in Second

Generation after Monohybrid Mating

Use Multiplication to Figure the Probability of the Inheritance of Alleles

A Testcross Also Can Reveal Genotypes

Testcross• Learn the genotype of a (nonhuman) organism• Cross organism with homozygous recessive

organism (aa)• If all offspring are Aa, parent was probably AA• If some of the offspring have the dominant trait and

some have the recessive trait, parent was Aa

19.4 How Genes for Different Traits Are Sorted into Gametes

When we consider more than one trait, we see that the gene for each trait is inherited independently of the gene of other traits

How Genes for Different Traits Are Sorted into Gametes

Independent assortment• Occurs during meiosis• A given chromosome and its genes move

randomly into gametes• Metaphase I• Metaphase II

Crosses between individuals heterozygous for two traits yields sixteen different gamete unions• Probability displayed using a Punnett square

Independent Assortment: Chromosomes Moved at Random into Forming Gametes

Fig. 19-8, p. 378

One of two possible alignments The only other possible alignment

a Initial chromo- some alignments (at metaphase I):

b The resulting alignments at metaphase II:

c Possible combinations of alleles in gametes: AB ab Ab aB

Stepped Art

Tracking Two Traits Shows the Results of Independent Assortment

Fig. 19-9, p. 379

CcDd CcDdmeiosis,

gamete formationmeiosis,

gamete formation

1/4 CD

1/4 Cd

1/4 cD

1/4 cd

1/4 CD

1/16 CCDD

1/16 CCDd

1/16 CcDD

1/16 CcDd

1/4 Cd

1/16 CCDd

1/16 CCdd

1/16 CcDd

1/16 Ccdd

1/4 cD

1/16 CcDD

1/16 CcDd

1/16 ccDD

1/16 ccDd

1/4 cd

1/16 CcDd

1/16 Ccdd

1/16 ccDd

1/16 ccdd

Adding up the combinations possible:

9/16 or 9 chin fissure, dimples3/16 or 3 chin fissure, no dimples

3/16 or 3 smooth chin, dimples1/16 or 1 smooth chin, no dimples

Probability Rules Apply to Independent Assortment

19.5 Single Genes, Varying Effects

Some traits have clearly dominant and recessive forms

For most traits, however, the story is not so simple

One Gene May Affect Several Traits

Pleiotropy• Wide-ranging effect of one gene

Sickle-cell anemia• One amino acid substitution in hemoglobin

• Val instead of glu

• Pleiotropic effects• Treatments

Single Genetic Change Leads to Many Physical Effects of Sickle-Cell Anemia

Fig. 19-11a, p. 380

Fig. 19-11b, p. 380

In Codominance, More Than One Allele of a Gene Is Expressed

Codominance• Heterozygous for a trait, but both alleles are

expressed• Example: alleles for blood type determine presence

or absence of polysaccharides on surface of red blood cells

• IA and IB; codominant when paired with each other

Multiple allele system• A gene that has three or more alleles

There Are Several Possible Allele Combinations for ABO Blood Types

19.6 Other Gene Effects and Interactions

Many phenotypes, such as eye color, can’t be predicted with certainty

Biologists have uncovered several underlying causes for these variations

Other Gene Effects and Interactions

Penetrance• Probability that someone who inherits an allele

will have the phenotype associated with it

Cystic fibrosis• Homozygous recessive• 100% penetrant

Polydactyly• Dominant allele for extra digits• Incompletely penetrant

People with Polydactyly Have Extra Digits on Their Hands or Feet

Polygenic Traits Come from Several Genes Combined (1)

Polygenic traits• Combined expression of several genes• Skin and eye color; many variations due to the

amount and distribution of melanin

Continuous variation• Populations show a range of continuous

differences• Most evident in traits that can be measured, e.g.,

height

Polygenic Traits Come from Several Genes Combined (2)

Multifactorial trait• Phenotypes shaped by more than one gene and

affected by the environment, e.g., height

Eye Color Is Just One of Many Human Polygenic Traits

The Environment Can Affect Phenotypes

Height• Diet low in protein• Disease or injury that prevents the normal release

of growth hormone

Skin color• Tanning

Good lifestyle choices• May limit the chances that a harmful gene(s) will

be expressed

“Tongue-Roller” Trait Is Due to a Dominant Allele