Chapter 20

Patterns of Genetic Inheritance

Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.

Points to Ponder• What is the genotype and the phenotype of an individual?• What are the genotypes for a homozygous recessive and dominant

individuals and a heterozygote individual?• Be able to draw a punnett square for any cross (1-trait cross, 2-trait

cross and a sex-linked cross).• What are Tay-Sachs disease, Huntington disease, sickle-cell

disease, and PKU? • How are each of the above inherited?• What is polygenic inheritance?• What is a multifactorial trait?• What is sex-linked inheritance?• Name 3 X-linked recessive disorders.• What is codominance?• What is incomplete dominance?• What do you think about genetic profiling?

These traits are genetically inherited

Answer these questions about your inheritance:• Do you have a widow’s peak?• Are your earlobes attached or unattached?• Do you have short or long fingers?• Do you have freckles?• Can you roll your tongue?• Do you have Hitchhiker’s thumb?

20.1 Genotype and phenotype

GenotypeGenotype – specific genes for a particular trait

written with symbols– Alleles:

• alternate forms of a specific gene at the same position (locus) on a gene (e.g. allele for unattached earlobes and attached lobes)

• alleles occur in pairs

– Dominant gene: will be expressed and will mask a recessive gene (Tt or TT)

– Recessive allele: allele that is only expressed when a gene has two of this type of allele

Genotype

• Genotype– Homozygous dominant genotype:

• 2 dominant alleles (TT or AA)

– Homozygous recessive genotype: • 2 recessive alleles (tt or aa)

– Heterozygous genotype: • one dominant allele and one recessive allele

(Tt or Aa)

Phenotype

Phenotype – the physical or outward expression of the genotype

Genotype Phenotype

EE unattached earlobe

Ee unattached earlobe

ee attached earlobe

What are your genotype and phenotype?

20.1 Genotype and phenotype

Understanding genotype & phenotype

What about your inheritance?

Crosses• One-trait cross – considers the inheritance of one

characteristice.g. WW x Ww

• Two-trait cross – considers the inheritance of two characteristicse.g. WWTT x WwTT

• Gametes only carry one allele, so if an individual has the genotype Ww what are the possible gametes that this individual can pass on?Answer: either a W or a w but not both

Another example:

Punnett squares• Punnett squares

– use of a grid that diagram crosses between individuals by using the possible parental gametes

• These allow one to figure the probability that an offspring will have a particular genotype and phenotype

20.2 One-and Two-trait inheritance

Practicing punnett squares

• What would a punnett square involving a man (M) with a genotype Ff and a women (F) with a genotype Ff look like?

F – frecklesf – no freckles

20.2 One-and Two-trait inheritance

M/F F f

F FF Ff

f Ff ff

eggs

sper

m

Practicing ratios• Genotypic ratio:

– number of offspring with the same genotype

• Phenotypic ratio: – number of offspring with

the same outward appearance

• What is the genotypic ratio?1: 2: 1 (1 FF: 2 Ff: 1 ff)

• What is the phenotypic ratio? 3: 1 (3 with freckles and 1 with no freckles)

M/F F f

F FF Ff

f Ff ff

eggs

sper

m

Monohybrid crosses

20.2 One-and Two-trait inheritance

Monohybrid cross – an experimental cross in which parentsare identically heterozygous at one gene pair (e.g. Aa x Aa)

One Trait Crosses

Possible gametes for two traitsAll Possible Combinations of Chromosomes and

alleles in the gametes (via meiosis)

Dihybrid cross (a type of two-trait cross)

• Dihybrid cross– experimental cross usually involving parents that are

homozygous for different alleles of two genes and results in a 9:3:3:1 genotypic ratio for the offspring

Practicing a punnett square for 2-trait cross

• What would the punnett square look like for a dihybrid cross between a male that is WWSS and a female that is wwss?

Phenotypic Ratios of Common Crosses

Genotype Phenotype

Monohybrid x Monohybrid

(Ww x Ww)

3:1 (dominant to recessive)

Monohybrid x recessive

(Ww x ww)

1:1 (dominant to recessive)

Dihybrid x Dihybrid

(WwSs x WwSs)

9:3:3:1 (9 both dominant, 3 one dominant, 3 other dominant, 1 both recessive

Dihybrid x recessive 1:1:1:1 (all possible combinations)

Family Pedigrees for Genetic Disorders

• Autosomal Dominant– Individual with alleles AA or Aa will have disorder

• Autosomal Recessive– Only individuals with alleles aa will have disorder

• Key:– Square = male– Circle = female– Shaded circle/square = affected individual– Line between square and circle = union– Vertical line going downward = child/children

Autosomal recessive disorder• Individuals must be homozygous recessive to

have the disorder

Autosomal dominant disorder

• Individuals that are homozygous dominant and heterozygous will have the disorder

Autosomal Recessive Disorders

1. Tay-Sachs

2. Cystic Fibrosis

3. Phenylketonuria

4. Sickle-cell Disease

5. Huntington Disease

Genetic disorders of interest• Tay-Sachs disease:

– lack of the enzyme that breaks down lipids in lysosomes – results in membranous cytoplasmic bodies (MCB) in the

cells present in the cortical neuron– eventually death of a baby

• Cystic fibrosis: – Cl- do not pass normally through a cell membrane, so Na+

and water can not enter the cell– results in thick mucus in lungs and causes infections, clogs

pancreatic ducts preventing function of digestive enzymes

• Phenylketonuria (PKU): – lack of an enzyme needed to make a certain amino acid – affects nervous system development

Genetic disorders of interest

• Sickle-Cell disease: – red-blood cells are sickle shaped rather than

biconcave that clog blood vessels– Sickle-cell heterozygotes have sickle-cell traits in

which the blood cells are normal unless they experience dehydration or mild oxygen deprivation

• Huntington disease: – Caused by a mutated copy of the gene (on

chromosome 4) for a huntingtin protein resulting in too many glutamine amino acids

– leads to progressive degeneration of brain cells

Genetic disorders20.2 One-and Two-trait inheritance

Tay sachs

Cystic fibrosis

Huntington Disease

Polygenic inheritance• Polygenic traits - two or more sets of alleles govern

one trait – Each dominant allele codes for a product so these effects

are additive– Results in a continuous variation of phenotypes– Environmental effects cause intervening phenotypes – e.g. skin color ranges from very dark to very light– e.g. height vary among

• Multifactorial trait – a polygenic trait that is particularly influenced by the environment – e.g. skin color is influenced by sun exposure– e.g. height can be affected by nutrition

20.3 Beyond simple inheritance

Polygenic inheritanceDistribution of phenotypes expected to follow a

bell-shaped curve

Demonstrating environmental influences on phenotype

• Himalayan rabbit’s coat color influenced by temperature

• There is an allele responsible for melanin production that appears to be active only at lower temperatures

• The extremities have a lower temperature and thus the ears, nose paws and tail are dark in color

20.3 Beyond simple inheritance

Incomplete dominance

• Occurs when the heterozygote is intermediate between the 2 homozygotes

• Example: (curly hair) CC x SS (straight hair)

CS (wavy hair)

20.3 Beyond simple inheritance

Codominance

• Occurs when the alleles are equally expressed in a heterozygote

• Example:(Type A blood) AA x BB (Type B blood)

AB (Type AB blood that has characteristics

of both blood types)

20.3 Beyond simple inheritance

Multiple allele inheritance

• The gene exists in several allelic forms

• A person only has 2 of the possible alleles

• A good example is the ABO blood system

• A and B are codominant alleles

• The O alleles is recessive to both A and B therefore to have this blood type you must have 2 recessive alleles

20.3 Beyond simple inheritance

Multiple allele inheritance

Based on what you know what type of blood would each of the following individuals have in a cross between Ao and Bo?

possible genotypes: phenotypes: AB Type AB blood Bo Type B blood Ao Type A blood oo Type O blood

20.3 Beyond simple inheritance

Blood type inheritance20.3 Beyond simple inheritance

Sex-linked inheritance

• Traits are controlled by genes on the sex chromosomes X-linked inheritance:

allele is carried on the X chromosome

Y-linked inheritance: allele is carried on the Y chromosome

Most sex-linked traits are X-linked

20.4 Sex-linked inheritance

X-linked inheritance: Color blindnessCarried on the X chromosome

Cross:

XBXb x XBY

Possible offspring:XBXB normal vision female

XBXb normal vision female

XBY normal vision male

XbY normal vision male

20.4 Sex-linked inheritance

X-linked disorders

• More often found in males than females because recessive alleles are always expressed

• Most X-linked disorders are recessive:– Color blindness:

• most often characterized by red-green color blindness

– Muscular dystrophy: • characterized by wasting of muscles and death by age 20

– Hemophilia: • characterized by the absence of particular clotting factors

that causes blood to clot very slowly or not at all

X-linked disorders

X-linked disorders: Hemophilia

Bioethical focus: Genetic profiling• Genetic profiling is a way to look for genetic

disorders that you may have now or in the future– Discrimination concerns:

• Could insurance companies use this to increase rates or not insure you?

• Could an employer not hire you based on this knowledge?

– Employer concerns:• Could the government mandate they provide an environment for

each employee’s need in order to prevent illness or should employees be required to move from an area or job that could cause future disease?

– Public benefits:• Most believe this could lead to better preventative care• Having this type of information can allow complex studies that can

further our understanding of disease

20.4 Sex-linked inheritance