Chapter 8- Mendel and Heredity Mrs. CookBiology

Part One: Gregor Mendel 1843 – Gregor Mendel, a Austrian monk, tended

to the garden. 1851 – he goes to college to study science and

math His knowledge of math, most importantly

statistics, later helped him in his research on heredity – the transmission of characteristics from parents to offspring.

He studied many plants, but is remembered for his work on the garden pea plant, Pisum sativum

What did Mendel discover while working with his pea plants? Seven characteristics, which occurred in two contrasting traits.

A trait is a genetically determined variant of a characteristic

7 Characteristics

How did he work with the plants? He controlled their pollination. Pollination occurs when pollen grains

produced in male reproductive parts of a flower, called anthers, are transferred to the female reproductive part of the flower, called the stigma.

Self-pollination occurs when pollen from one flower lands on the stigma from that same flower.

Cross-Pollination occurs between flowers of two different plants.

MENDEL MANUALLY CROSS-POLLINATED TO LOOK AT SPECIFIC TRAITS AND MAKE OBSERVATIONS ON HOW THOSE TRAITS APPEARED IN OFFSPRING

MENDEL’S EXPERIMENTS: He began with true-breeding plants. True-Breeding, or pure, for a trait means

that they produce offspring only with that trait when they self-pollinate. He called his true-breeding parents his P generation

He then cross-pollinated two true-breeding plants of contrasting traits and observed the offspring. The offspring of this cross were called the F1 Generation or first filial generation.

He then let the F1 generation self-pollinate and observed the offspring. This was called the F2 Generation or second filial generation.

Reasons to use Peas 1. Easy to grow 2. Mature quickly 3. Large # Offspring 4. Can self-pollinate 5. Few traits – easy to see 6. Short generation time

Mendel’s Results and Conclusions: He found that in

every F1 generation, one trait disappeared in the offspring.

BUT, in the F2 generation, that trait returned, but always in a 3:1 ratio.

Recessive and Dominant Traits

A dominant trait is one that will mask, or dominate, over the other trait in the pair.

A recessive trait is one that will be masked, by the dominant gene

Support for Mendel’s Conclusions:

Molecular Genetics is the study of the structure and function of chromosomes and genes.

Each of two or more alternative forms of a gene is called an allele.

Letters are used to represent allelesCapital Letters represent Dominant alleles

Lowercase letters represent Recessive alleles.

The Law of Segregation The Law of Segregation states that

a pair of alleles is segregated, or separated, during the formation of gametes.

That each gamete only has one form of each allele

and when the two gametes join, a complete set is created.

The Law of Independent Assortment

The Law of Independent assortment states that alleles separate independently of one another during the formation of gametes.

Alleles of individual characteristics are not connected, due to the random separation of homologues during Meiosis**applies to genes on different chromosome pairs!

An organism’s genetic makeup is its genotype. Are the alleles that the organism

inherits from its parents. Genotype is always written as

letters!

An organism’s appearance is its phenotype Are also inherited from both

parents, but different combinations can produce different appearances.

Does not always indicate genotype! Not only can recessive alleles be

masked, but environmental factors can change the phenotype.

When both alleles are alike, the organism is said to be homozygous for that characteristic. Can be homozygous dominant or

homozygous recessive. Examples: T T or t t

When the two alleles are different, that organism is said to be heterozygous for that trait.

Example: T t

Probability is the likelihood that a specific event will occur. Can be expressed as a decimal, a percentage, or a

fraction Probability is determined by the following equation:

Probability = number of times an event is expected to happen

number of times an event could happen

The results of probability are more likely to occur when there are many trials.Example: Tossing a coin to get heads or tails

Punnett Square A cross involving only one trait is called

a monohybrid cross The offspring of a monohybrid cross

are called monohybrids Biologists use a diagram called a

Punnett Square to make predictions about the possible outcomes of genetic crosses. Predicts the Chances! From the Punnett Square, you can get

two types of ratios:

Genotypic Ratio Has three numbers: The first

represents the number that can show homozygous dominant, the second the number that can show heterozygous, and the third is the number that can show homozygous recessive.

Phenotypic Ratio Has two numbers: The first

represents the number of individuals that can show/express the dominant phenotype and the second number represents the number of individuals that can show/express the recessive trait.

Testcross A testcross is when an individual of

unknown genotype is crossed with a homozygous recessive individual.

A testcross can determine the genotype of any individual whose phenotype expresses the dominant trait.

Chapter 8- PART TWO-Inheritance of Traits Polygenic Traits: When several genes

influence a character Maybe scattered along the same

chromosome Or located on different chromosomes Some Examples: hair, eye color, skin color,

and height

Incomplete Dominance Multiple Alleles Codominance Sex-linked

Incomplete Dominance In Mendel’s experiments, one allele was

completely dominant over the other. That is called complete dominance.

Incomplete Dominance though is when the phenotype of the heterozygous individual is an intermediate between the phenotypes of the dominant and recessive traits. Four O’Clock Flowers

Multiple Alleles- Codominance Codominance occurs when both

alleles for a gene are expressed in a heterozygous individual.

Neither allele is dominant or recessive, nor do the alleles blend in this phenotype.

Mutiple Alleles & Co-dominance ABO Blood Types- More than 2 alleles

for a gene. A B AB O

Blood Type (Phenotype)

Possible Genotypes

A IAIA OR IAi (IA IS A DOMINANT ALLELE)

B IBIB OR IBi (IB IS A DOMINANT ALLELE)

AB IAIB (TOGETHER, IA AND IB EXHIBIT CODOMINANCE)

O ii (i IS THE RECESSIVE ALLELE) (LITTLE i)

The type of ANTIGEN (MARKER) on a person’s blood cells determines the BLOOD TYPE.

The antigen is a FACTOR that causes CLOTTING.

A blood TRANSFUSION can only work if the blood from the DONOR and the RECIPIENT MATCH.

Mixing blood types will cause a CLUMPING of the blood cells called AGGLUTINATION.

Group O is called the UNIVERSAL DONOR.

Group AB is called the UNIVERSAL RECIPIENT.

RH FACTOR is another type of ANTIGEN. RH+ has the antigen. RH- DOES NOT.

This causes problems in the SECOND BORN child, where the mother is RH- and the father is RH+. Results in hemolytic disease of the newborn. Mother’s get a RhoGAM shot during 1st

pregnancy and after delivery.

Sex-linked Traits SEX-LINKED GENES AND TRAITS

The term SEX-LINKED TRAIT refers to a trait that is coded for by an ALLELE on a SEX CHROMOSOME

The X-chromosome is much larger and can carry MORE ALLELES, so there are MORE X-LINKED than Y-LINKED traits.

Most X-LINKED ALLELES have NO counterpart on the Y CHROMOSOME.

Because MALES have only one X chromosome, a male who carries a RECESSIVE ALLELE on the X chromosome will exhibit the sex linked trait. Therefore, you will most likely see sex-linked traits in MALES because females have a better chance of a dominant gene masking the recessive gene.

Men always inherit sex-linkd traits from their MOTHER on the X-chromosome. EXAMPLES: COLORBLINDNESS AND HEMOPHILIA.

SEX-INFLUENCED TRAITS – traits that are generally associated with one sex, but the genes are carried on the AUTOSOMES rather than the sex chromosomes.

These are influenced by sex hormones: ESTROGEN and TESTOSTERONE

EXAMPLES – BALDNESS, FACIAL HAIR.

Single Allele TraitsWhen a trait or disease is caused by a

SINGLE ALLELE.Scientists have discovered more than

300 Human traits caused by a SINGLE DOMINANT ALLELES

HUNTINGTON’S DISEASE is an autosomal dominant disease

CLEFT CHIN, FRECKLES, FREE EARLOBE

For more than 250 traits, genetic disorders occur when the individual receives two recessive alleles

Phenylketonuria (PKU)A person lacks the enzyme to break down the chemical PHENYLALANINE.

PHENYLALANINE builds up in the brain and destroys brain cells, causing severe mental retardation.

BABIES who test positive for PKU are given a special diet low in phenylalanine.

Sickle Cell AnemiaCauses HEMOGLOBIN to be transformed so that Red blood cells are produced in the shape of a SICKLE instead of a flattened sphere.

Red blood cells clog vessels and organs causing oxygen deprivation.

More common in persons of African descent.

Tracking Traits Predicting Results of Dihybrid

Crosses A Dihybrid Cross is a cross in

which two characteristics are tracked at the same time.

The offspring of a dihybrid cross are called dihybrids

B= Brown

b= Blond S=

straight s= curly

How we study Inheritance- Pedigree Analysis Pedigree: a family history that shows

how a trait is inherited over several generations. Particularly helpful if the trait causes a

genetic disorder and the family members want to know if they are carriers or if their children might get the disorder.

Pedigree Rules In a pedigree, Squares stand for men and

Circles stand for women. Filled symbol means the person has the

trait or condition An empty symbol means the person does

not have the trait or conditionThese individuals can be CARRIERS. A carrier has one copy of the recessive allele but do not have the disease. A carrier may be denoted by a half filled symbol.

A HORIZONTAL LINE joining make and females indicates mating (not necessarily marriage)

A VERTICAL LINE indicate offspring arranged from left to right in order of their birth.

ROMAN NUMERALS label different generations.

Cystic Fibrosis Most common, fatal, hereditary,

autosomal recessive, disorder among Caucasians.

1/25 of Caucasian individuals has at least 1 copy of a defective gene that makes a protein necessary to move chloride into and out of cells.

1/2,500 Caucasian infants in the US is homozygous for the CF allele.

The airways of the lungs become clogged with mucus, and the ducts of the liver and pancreas become blocked.

Dr.s can treat the symptoms but there is no known cure.

Who Has CF, who are carriers?

Hemophilia X-linked, recessive disorder Impairs the blood’s ability to clot. More than a dozen genes code for the

proteins involved in blood clotting. Females are usually carriers, mostly

affects Males because they only need 1 copy of the gene to get the disorder.

Huntington’s Disease Autosomal Dominant disease First symptoms- mild forgetfulness and

irritability Appear in persons in their 30’s or 40’s HD causes loss of muscle control,

uncontrollable physical spasms, severe mental illness, and eventually death

Most do not know they have the disease until after they have children

Other Genetic Disorders Genes play a role in some medical

conditions that were once thought to caused only by the environment: Diabetes Alzheimer’s Hypertension –heart disease Parkinson’s disease Breast Cancer

Other Forms of Genetic Disorders

Mutation A change in genetic code caused by

mutagens such as cancers, radiation, medications, drugs, alcohol, viruses, environmental factors Can be positive, negative or neutral

Albinism Recessive, lack of pigments (colors)

IN NONDISJUNCTION a chromosome fails to separate from its HOMOLOGUE during meiosis; one gamete receives an extra copy of that chromosome and the other gamete receives none.

An example is DOWN SYNDROME (TRISOMY 21) where the individual has an extra chromosome 21.

TRISOMY – A third or extra chromosome on the pair

MONOSOMY – missing a chromosome of the pair.

Klinefelter’s Syndrome Trisomy – Extra X Males – XXY

“femaleness” – enlarged breasts Underdeveloped body hair Small testes Some mental retardation

Females – XXX Some M.R. Erratic Menstruation May be sterile

Genetic Testing and Gene Therapy

GENETIC COUNSELING is the process of informing a person or couple about their genetic makeup

Is a form of medical guidanceIdentifies risk of defectsHelps to decide whether to have children

Uses pedigrees

For many you can treat the symptoms, like with the genetic disease PKU or Cystic Fibrosis

Others can have Symptom -Prevention Measures; Example: giving Insulin to a person with diabetes.

GENE THERAPYYou can REPLACE a gene called GENE THERAPY. You can replace a bad one with a good one.

Relies on knowing where all the genes are located on the chromosome.

Researches are still working to increase the effectiveness of gene therapy

Would you want to know? CBS Sunday Morning- November 2012

http://www.cbsnews.com/8301-3445_162-57556650/a-need-to-know-the-worst-news-you-will-ever-hear/