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Heredity Biology Honors

Mendel laid the groundwork for genetics.

• Traits are distinguishing characteristics that are inherited.

• Genetics is the study of biological inheritance patterns and variation.

• Gregor Mendel showed that traits are inherited as discrete units.

• Many in Mendel’s day thought traits were blended.

Mendel’s data revealed patterns of inheritance.

• Mendel made three key decisions in his experiments.

– use of purebred plants

– control over breeding

– observation of seven “either-or” traits

Mendel used pollen to fertilize selected pea

plants.

Mendel controlled the

fertilization of his pea plants

by removing the male parts,

or stamens.

He then fertilized the female

part, or pistil, with pollen from

a different pea plant.

– P generation crossed to produce F1 generation

– P generation: Parental

– F1: First filial; first generation

– F2: Second filial; second generation

– Interrupted the self-pollination process by removing male

flower parts

Mendel allowed the resulting plants to self-

pollinate.

– Among the F1 generation, all plants had purple flowers

– F1 plants are all heterozygous

– Among the F2 generation, some plants had purple

flowers and some had white

• What’s self pollination?

Mendel observed patterns in the first and

second generations of his crosses.

Mendel’s conclusions.

•Traits are inherited as discrete units. •Law of Segregation-

•Organisms inherit two copies of each gene, one from each parent. •Organisms donate only one copy of each gene in their gametes. The two copies of each gene segregate, during gamete formation.

purple white

Traits, Genes, and Alleles- Terms to Know

• Gene- piece of DNA that provides a set of instructions to a cell to make a protein.

• Allele- any alternative form of a gene that may occur at a specific locus.

• Homozygous- alleles are identical to each other.

• Heterozygous- alleles are different from each other.

• Genotype- genetic makeup

• Phenotype- physical makeup

• Dominant- expressed allele (Hh / HH)

• Recessive- allele is expressed only when two copies are present (hh)

• Alleles can be represented using letters.

– A dominant allele is

expressed as a phenotype

when at least one allele is

dominant. Ex- DD or Dd

– A recessive allele is

expressed as a phenotype

only when two copies are

present. Ex- dd

– Dominant alleles are

represented by uppercase

letters; recessive alleles by

lowercase letters.

Let’s Practice • For each genotype below, indicate whether it is heterozygous (He) or

homozygous (Ho) • AA • Ee • Ii • Mm • Bb • ff • Jj • nn • Cc • Gg • kk • oo • DD • HH • LL • Pp

For each of the genotypes below determine

what phenotype would be possible.

Purple flowers are dominant to white flowers.

PP

Pp

pp

Brown eyes are dominant to blue eyes

BB

Bb

bb

Bobtails in cats are recessive.

TT

Tt

tt

Round seeds are dominant to wrinkled seeds

RR ____ Rr ______________

rr ______________________

Punnett squares illustrate genetic crosses.

• The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. – The axes represent

the possible gametes of each parent.

– The boxes show the possible genotypes of the offspring.

• The Punnett square

yields the ratio of

possible genotypes

and phenotypes.

A monohybrid cross involves one trait.

• Monohybrid crosses examine the inheritance of only one specific trait.

– homozygous dominant-DD

– homozygous recessive- dd

– heterozygous- Dd

Homozygous vs. Homozygous

• Parents are HH X hh.

• All heterozygous offspring.

Heterozygous- Heterozygous

• Parents are Ff X Ff

• Offspring outcomes:

– 1:2:1 Genotypic ratio

– 3:1 Phenotypic ratio

Heterozygous- Homozygous

• Parents are ff X Ff

• Offspring

– Genotypic ratio: 1:1 or 50% / 50%

– Phenotypic ratio: 1:1 or 50% / 50%

A dihybrid cross involves two traits

• Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio.

• Mendel’s dihybrid

crosses led to his

second law,

the law of independent

assortment.

• The law of

independent

assortment states that

allele pairs separate

independently of each

other during meiosis.

How do we figure out the alleles?

• The FOIL method!

• See the board and record in your notes!

Let’s Practice!

Review

• 1. Which type of reproduction produces the most genetic diversity?

• 2. True of False. There are about 70 trillion different combinations of chromosomes when a couple has a baby?

• 3. What process increases genetic diversity in meiosis 1 – prophase 1?

• 4. During which phase of meiosis does independent assortment occur?

Chromosomes and Phenotypes

Many factors affect phenotype!

• Different alleles can produce different phenotypes.

• Most traits in humans are a result of autosomal genes.

• Genetic disorders caused by recessive alleles. – Two copies of recessive alleles must be present. – Parents are typically heterozygous, therefore they are

carriers (Hh). – Carriers do not show disease symptoms, but can pass

them on to the offspring. – Example- Cystic fibrosis.

Dominant Phenotypes

• Far less common.

• Ex- Huntington’s Disease: damages and breaks down the nervous system and usually appears during adulthood. Fatal.

– If both parents are heterozygous, there is a 75% chance the offspring will have the disease.

– Chromosome 4

Sex Linked Genes

• Genes located on the sex chromosomes.

• Female= XX Male = XY

• Only females can be the carriers of sex linked disorders, therefore if a male has a sex linked disorder, he got it from mom!

• X chromosome has about 1100 known genes.

• Y chromosome has about 250 known genes.

• Examples: Hemophilia (blood is missing the protein to clot), Duchenne’s muscular dystrophy, and Fragile X Syndrome.

Muscular Dystrophy

• Progressive muscle weakness and degeneration.

• Caused by the absence of the protein dystrophin, which allows the muscles to stay intact.

Fragile X

• Symptoms include delays in talking, anxiety, and hyperactive behavior. Some people have seizures. Physical features might include large ears, a long face, a prominent jaw and forehead, and flat feet.

Let’s Practice

• See worksheet in your notes!

• https://www.youtube.com/watch?v=h2xufrHWG3E

Complex Patterns of Inheritance

• Incomplete Dominance – Neither allele is

completely dominant nor recessive.

– Ex- four o’clock plant: one plant is homozygous dominant for red color, while the other is homozygous white; resulting in a pink flower.

Codominance

• Both alleles of a gene are expressed equally.

• Neither is dominant nor recessive.

• Ex- Black rabbit mates with a white rabbit; the offspring are black and white.

Codominance in Blood Types

• ABO blood types • Multiple allele trait

because it has three different alleles.

• IA , IB , i • A and B result in an

antigen, a protein, on the surface of the red blood cells.

• i is recessive = no antigen.

• IA and IB are codominant.

Polygenic Traits

• Traits produced by two or more genes.

• Ex- human skin color or eye color

Epistasis • One gene interferes with the

expression of other genes • Ex- Albinism: a single epistatic

gene interferes with the expression of other genes.

• Polygenic trait- fur color • Ex- Mice: five different genes

that interact to produce the phenotype. – Two give its general color – One affects the shading – One determines if it will have

spots – Fifth one can overshadow all of

the others

Video review

• https://www.youtube.com/watch?v=YJHGfbW55l0

• https://www.youtube.com/watch?v=9O5JQqlngFY

Tracing genes in a family

• Pedigree- chart that can trace the phenotypes and genotypes in a family to determine whether people carry recessive alleles.

Helpful Hints

• How do I know if the gene is on the sex chromosome or an autosome?

• If the same number of males and females have phenotype = gene is an autosome.

• If the phenotype is more common in males, then the gene is mostly likely on the X chromosome.

Karyotypes

• Picture of your chromosomes.

• Down Syndrome= Trisomy 21. Three chromosomes on number 21.

• Human Genome Project: completed in April 2003.

• https://www.youtube.com/watch?v=MvuYATh7Y74

Sickle Cell Anemia- #11

• Prevalent among African Americans

• One in 12 African Americans carry the trait

• One in 100 Hispanics carry the trait

• https://www.genome.gov/10001219/learning-about-sickle-cell-disease/

Trisomy 18- Edwards Syndrome

10% of children born with this syndrome live to be 10 years old.

Klinfelter Syndrome • Male is born with an extra x

• Sex chromosomes are 47, XXY

• Affects physical and cognitive development, small testes, low testosterone levels, breast enlargement, reduced secondary sex characteristics, and infertility.

• Some affected individuals also have genital differences including undescended testes (cryptorchidism), the opening of the urethra on the underside of the penis (hypospadias), or an unusually small penis (micropenis).

• https://ghr.nlm.nih.gov/condition/klinefelter-syndrome

How does Klinefelter syndrome occur?

• Klinefelter syndrome and its variants are not inherited; these chromosomal changes usually occur as random events during the formation of reproductive cells (eggs and sperm) in a parent. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes.

Turner Syndrome • Female is born with one X

• 45,X

• Short stature and loss of ovarian function

• Many girls do not undergo puberty unless they have hormone injections.

• About 30 percent of females with Turner syndrome have extra folds of skin on the neck (webbed neck), a low hairline at the back of the neck, puffiness or swelling (lymphedema) of the hands and feet, skeletal abnormalities, or kidney problems. One third to one half of individuals with Turner syndrome are born with a heart defect, such as a narrowing of the large artery leaving the heart or abnormalities of the valve that connects the aorta with the heart (the aortic valve). Complications associated with these heart defects can be life-threatening.

• Most girls and women with Turner syndrome have normal intelligence. Developmental delays, nonverbal learning disabilities, and behavioral problems are possible, although these characteristics vary among affected individuals. https://ghr.nlm.nih.gov/condition/turner-syndrome