Patterns of Inheritance

Chromosome Review

Genetics

• Study of the patterns of inheritance• Mendelian Genetics

– Gregor Mendel– Pea plant experiments

• Grow easily• Distinguishable characteristics

– Round/Wrinkly, Yellow/Green, Tall/Short

• Can control mating

Terminology

• Allele—Alternate forms of a gene• Dominant Allele—trait that exerts its effects

whenever present– Symbolized by capitol letter (B)– Ex. Dwarfism, Huntington’s disease

• Recessive Allele—trait that is masked if a dominant trait is present– Symbolized by lower case letter (b)– Ex. Blue eyes

Terminology

• Genotype—Genetic makeup of an organisms– Homozygous—2 alleles that are the same (BB or bb)– Heterozygous—2 different alleles (Bb)

• Phenotype—Observable characteristics– Brown eyes, brown hair, tall, short

• Generation—group of organisms born around the same time– P Generation—Parental– F1—offspring of parent, first filial generation– F2—1st generation of offspring

Genetic TerminologyTerm Definition

GenerationsP First (parental) generationF1 children (Latin filii) of

generation PF2 children of generation F1

Chromosomes and Geneschromosome macromolecules found in

cells, consisting of DNA, protein and RNA

gene the unit of heredity of a living organism

locus specific location of a gene, DNA sequence, or position on a chromosome

allele one of a number of alternative forms of the same gene or genetic locus

Term Definition

Dominant and recessivedominant allele an allele that is always expressed if presentrecessive allele an allele that is only expressed if the dominant

allele is not present

Genotypes and Phenotypes genotype the genetic makeup of a cell, an organism, or

an individual homozygous contains two of the same allele of a gene

heterozygous contains two different alleles of a gene

phenotype an organism's observable characteristics or traits

true breeding pure-bred, is an organism that always passes down a certain phenotypic trait to its offspring

wild type most common phenotype in naturemutant physical or genetic difference from a mutation

in a gene

Tracking Inheritance

• Monohybrid Cross—Mating between individuals that are BOTH heterozygous

• Dihybrid Cross—Mating between individuals that are heterozygous for 2 traits (genes)

• Punnett Square—diagram showing genotypes in a monohybrid cross– Test cross—a Punnett square done to figure out the

genotype of an unknown• Always use a homozygous recessive individual with

unknown

Mal

e pa

rent

Female parent

Yy

Yy

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Y y

Female gametes (1:1)

Y

y

YY

Yy yy

Yy

Genotypic ratio 1:2:1 (1 YY: 2 Yy: 1 yy)Phenotypic ratio 3:1 (3 yellow: 1 green)

Mal

e ga

met

es (1

:1)

Monohybrid Cross

y y

Y

y

Y

Mal

e ga

met

es

y

Y

y

y

Mal

e ga

met

es

y

If plant is homozygous dominant (YY):

YY

Female gametes

Yellow seeds (Yy): 100% chanceYy

Yy

Yy

Yy

yy

If plant is heterozygous (Yy):

Yy

Female gametes

Yellow seeds (Yy): 50% chanceGreen seeds (yy): 50% chance

Yy yy

yyYy

Law of Segregation

• Two alleles of each gene are packed into separate gametes– Alleles move apart during meiosis– Occurs during meiosis 1—Metaphase

• Homologous chromosome move to opposite ends of poles

Law of SegregationEvery individual contains a pair of alleles for each particular trait which segregate or separate during cell division (assuming diploidy) for any particular trait and that each parent passes a randomly selected copy (allele) to its offspring.

Gametes will be paired randomly with gametes from the other parent.

Example1. P generation2. F1 generation3. F2 generation

Genetics Problem

The ability to curl your tongue up on the sides (T, tongue rolling) is dominant to not being able to roll your tongue. A woman who can roll her tongue marries a man who cannot. Their first child has his father’s phenotype. What are the genotypes of the mother, father, and child? What is the probability that their second child will be a tongue roller?

Law of Independent Assortment

• The segregation of alleles of one gene does not influence the alleles for another gene– Alleles are RANDOMLY packaged in gametes

• Since we have SO MANY genes, can’t keep doing punnett square.– Use Product Rule—The chance that 2 independent

events will both occur = the product of individual chances that each event will occur

Product Rule

Law of Independant Assortment

About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What percentage of the children will be albinos? What percentage of the children will be non-taster of PTC?

Law of Independent Assortment and Dihybrid Crosses

Recessive Disorders

• Recessive Disorder—disorder is ONLY expressed when both alleles are recessive– Can be “carried”– Heterozygous individuals

can carry gene but not express disease

Dd Hearing carrier father

Dd Hearing carrier mother

D d

D DDHearing

DdHearing (carrier)

d Dd Hearing (carrier)

dd Deaf

Recessive Disorders

Dominant Disorders

• Dominant Disorder– Condition is expressed when

an individual has at least one dominant allele

Achondroplasia

Autosomal DisordersDisorder Likelihood Symptoms

Recessive DisordersAlbinism approximately 1

in 17,000Most people with albinism appear white or very pale. Because individuals with albinism have skin that entirely lacks melanin, their skin can burn more easily from overexposure

Galactosemia approximately 1 in 60,000 (Caucasians)

In galactosemic patients, galactitol accumulates in body tissues. This can result in an enlarged liver, cirrhosis, renal failure, cataracts, brain damage, and ovarian failure. Without treatment, mortality in infants with galactosemia is about 75%.

Phenylketonuria (PKU)

approximately 1 in 15,000 (US)

Abnormally high levels of phenylalanine accumulate in the blood and are toxic to the brain. Complications intellectual disability, brain function abnormalities, mood disorders, irregular motor functioning, and behavioral problems.

Sickle-cell disease approximately 1 in 5,000 (US)

Sickle-cell disease may lead to various acute and chronic complications, several of which have a high mortality rate. Sickle-cell anaemia can lead to various complications, including: increased risk of infection, stroke, and chronic pain.

Tay–Sachs disease approximately 1 in 3,500 (Ashkenazi Jews)

Tay–Sachs disease is typically first noticed in infants around 6 months old displaying an abnormally strong response to sudden noises or other stimulus, known as the “startle response.” There may also be listlessness or muscle stiffness (hypertonia).

Dominant DisordersAlzheimer’s disease unknown dementia; memory loss and mental decay; not all cases are the result of genetics

Huntington’s disease approximately 5–10 in 100,000

a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and behavioral symptoms. It typically becomes noticeable in mid-adult life.

Hypercholesterolemia unknown the presence of high levels of cholesterol in the blood; can lead to atherosclerosis and other heart problems; can be caused by genetics or lifestyle

Variations on Mendel’s Laws

• Incomplete Dominance– Individual who is

heterozygous show intermediated phenotype

Variations on Mendel’s Laws

• Pleiotropy – One gene has many

effects

Variations on Mendel’s Laws

• Pleiotropy – One gene has many

effects

Sickle-cells can cause many problems in a person’s life, including organ damage, chronic illness, and pain.

Variations on Mendel’s Laws

Polygenic Inheritance—Many genes control one phenotype

Variations on Mendel’s Laws

Polygenic Inheritance—Many genes control one phenotype

Read more here: http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0000069

Sex Linked Genes

• Sex Determination– Female – XX– Male - XY

Sex Linked Genes

• Can be carried on either X or Y chromosome– More common on X chromosome

• Affect more males than females

XAXa XAYFemale Male

Sex Chromosomes and Sex Linked Genes

• Eye color in fruit flies

X-Linked DisordersDisorder Genetics Characteristics

X-Linked Recessive InheritanceDuchenne muscular dystrophy mutation in the dystrophin gene early life muscle degeneration and eventual death

Fragile X syndrome expansion of the CGG trinucleotide repeat on the X chromosome

most widespread single-gene cause of autism and inherited cause of intellectual disability, especially among boys

Hemophilia A mutant allele; ~70% of the time it is an X-linked recessive trait, but ~30% of cases arise from mutations

deficiency in clotting factor VIII, which causes increased bleeding and usually affects males

Red–green color blindness absence or mutation of the genes for red or green color receptors

difficulty with discriminating red and green hues

Rett syndrome mutations in the gene MECP2 located on the X chromosome

typically have no verbal skills, and about 50% of individuals affected do not walk. Scoliosis, growth failure, and constipation are very common and can be problematic.

X-Linked Dominant Inheritance Hypertrichosis unknown abnormal amount of hair growth over the bodyX-linked hypophosphatemia mutation in the PHEX gene

sequence (Xp.22) and subsequent inactivity of the PHEX protein

It can cause bone deformity including short stature and genu varum (bow leggedness).

Retinitis pigmentosa (some forms)

unknown degenerative eye disease that causes severe vision impairment and often blindness