Chapter 7: Mendel & HeredityKnowledge of heredity explains why you inherit certain genes that affect your…________________,________________,and _____________

Objectives Vocabulary• Identify the investigator whose studies formed the

basis of modern genetics.• List characteristics that make the garden pea a

good subject for genetic study.• Summarize the three major steps of Gregor

Mendel’s garden pea experiments.• Relate the ratios that Mendel observed in his

crosses to his data.• Describe the four major hypotheses Mendel

developed.• Define the terms homozygous, heterozygous,

genotype, and phenotype.• Compare Mendel’s two laws of heredity.

HeredityGeneticsMonohybrid crossTrue-breedingP generationF1 generationF2 generation

AlleleDominantRecessiveHomozygousHeterozygousGenotypePhenotypeLaw of SegregationLaw of Independent Assortment

Mendels Studies of Traits

• Many of your _____________, including the color and shape of your eyes, the texture of your hair, and even your height and weight, ________________ those of your parents.

• The passing of traits from parents to offspring is called _________________.

• Long before the discovery of _______ and ________________, humans began experimenting with heredity

• From the beginning of recorded history, humans have attempted to alter __________ plants and ______________ animals to give them traits that are more useful to us.

Mendels Breeding Experiments

• The scientific study of heredity began more than a century ago with the work of an Austrian monk named Gregor Johann ________________.

• The hereditary patterns that Mendel discovered form the basis of _______________, the branch of biology that focuses on heredity.

• Mendel repeated experiments carried out earlier by a British farmer named T. A. ___________, who crossed purple and white pea plants

• The term cross refers to the _____________ or ____________ of two individuals

• Mendel’s experiments differed from Knight’s because Mendel ___________ the number of each kind of offspring and _______________ the data

• To control breeding, Mendel cut out the male reproductive organs, the ________

• from one flower in the cross

• Only the female reproductive organ, the ___________, remained

• Mendel then used a brush to transfer the male gamete, commonly called ______________ to the ___________ of another flower

Useful Features in Peas

The garden pea was a good subject for studying heredity for several reasons:

1. Several traits of the garden pea exist in ________ clearly different forms.

2. The male and female reproductive parts of garden peas are enclosed within the _________ flower. This allows cross pollination or ________ pollination

3. The garden pea is _________, grows easily, matures quickly, and produces _____________ offspring.

Traits Expressed as Simple Ratio

• Mendel’s initial experiments were ____________ crosses.

• A monohybrid cross is a cross that involves ______ pair of contrasting traits.

• For example, crossing a plant with purple flowers and a plant with white flowers is a monohybrid cross.

– The single trait studied is flower ________

– What were the two contrasting forms?

Mendel Carried Out his Experiments in 3 Steps

Step 1 Mendel allowed each variety of garden pea to ________-pollinate for several generations to ensure that each variety was _______-breeding for a particular trait. “True breeding” means that all of a plant’s offspring display only ________ form of a trait. These true-breeding plants are considered _______________ and served as the ___________ or _____ generation in Mendel’s experiments.

Step 2 Mendel then _________ -pollinated two P generation plants that had _____________ forms of a trait, such as purple flowers and white flowers. Mendel called the offspring of the P generation the first _____________ generation, or ____ generation. filial = son or daughter in latin

• What color flowers did all of the F1 offspring have?

• Are the F1 offspring still purebreds?

Step 3: Mendel allowed the F1 generation to ______ -pollinate. He called the offspring of the F1 generation plants the

second filial generation, or ____ generation. What two colors showed up in the F2 offspring?

How many total F2 offspring did Mendel analyze?

What color ratio did they exhibit?

• The reduced ratio is very close to __________705 : 224 224 224

Mendels Crosses & Results

Mendel repeated this breeding experiment with __________________ other traits and acquired ______________ data

In Summary

• Each of Mendel’s F1 plants showed only ______ form of the trait.

– What color were they?

• But when the F1 generation was allowed to self-pollinate, the missing trait _____________ in some of the plants in the F2 generation.

– What was the missing color?

• For each of the seven traits Mendel studied, he found a _______ ratio of contrasting traits in the F2 generation.

A Theory of Heredity

• Mendel correctly concluded from his experiments that each pea has _______ separate “heritable factors” for each trait—one from each parent.

• When gametes (sperm and egg cells) form, each receives only ______ of the organism’s two factors for each trait.

• But when gametes _______ during fertilization, the offspring has two factors for each trait, one from each parent. Today these factors are called __________.

Mendels Hypotheses

The four hypotheses Mendel developed as a result of his experiments now make up the Mendelian theory of heredity—the foundation of genetics.

1. For each inherited trait, an individual has _______ copies of the gene—one from each parent.

2. There are ____________ versions of genes. Today the different versions of a gene are called its ________

3. When two different alleles occur together, one of them may be completely _______________, while the other may have no ________________ effect on the organism’s appearance.

Mendel described the expressed form of the trait as _______________.

The trait that was not expressed when the dominant form of the trait was present was described as ____________________.

4. When gametes are formed, the alleles for each gene in an individual ______________ independently of one another. Thus, gametes carry only ______ allele for each inherited trait. When gametes unite during ________________, they combine to create an individual that has _____ alleles for each inherited trait.

Mendels Findings in Modern Terms

• Dominant alleles are indicated by writing the first letter of the trait as a ____________ letter. • Recessive alleles are also indicated by writing the first letter of the dominant trait, but the letter is

_______________.– Example: Purple is the dominant trait for the flower color of pea plants…

• The dominant allele is written as ______• The recessive allele is written as ______

• If the two alleles of a particular gene present in an individual are the __________, the individual is said to be __________________

– The prefix homo- means the same– An individual may be…

• Homozygous dominant = _____• Homozygous recessive = _____

• If the alleles of a particular gene present in an individual are ______________, the individual is _______________.

– The prefix hetero- means other or different– A heterozygous individual is written as ______

• In heterozygous individuals, only the _____________ allele is expressed; the recessive allele is present but __________________.

• What color flowers will the following pea plant have…Pp?• The set of alleles that an individual has is called its _________________________.• The physical appearance of a trait is called a _____________________.• Phenotype is determined by which ____________ are present.

– So if a plant has the genotype _____, what phenotype will it have?The Laws of Heredity

• Mendel’s findings allowed him to write two laws that accurately describe the behavior of chromosomes during _____________

• These laws are called…

– The law of _____________________________:This law states that the two alleles for a trait segregate or _______________ when gametes are formed. The alleles segregate because they are found on __________________ which separate as they are passed on to _____________

– The law of ______________ _______________________________: This law states that the alleles of different genes separate _______________ of one another during gamete formation. Mendel based this law on the fact that for the traits he studied, the inheritance of one trait did not ________________ the inheritance of any other trait. So for example, the inheritance of one trait, such as plant _____________, did not affect the inheritance of another trait such as flower ___________

• Remember, that the chromosomes line up randomly during metaphase ____ of meiosis.• As the chromosomes below separate, what traits might a gamete contain ?

• When Mendel wrote his laws, chromosomes had _______ yet been discovered…he also did not know that genes are ________ on chromosomes

• Using the chromosomes below, if a gamete receives the dominant P allele, what other allele will it receive?

• Genes that are ______________ away from each other on a chromosome are more likely to cross over without each other, while closer genes are ___________

• So while crossing over, independent assortment, and random fertilization all ____________ genetic variation, gene linkage ____________ the number of possible gene combinations

1. Identify the investigator whose studies formed the basis of modern genetics.

2. List characteristics that make the garden pea a good subject for genetic study.

3. Summarize the three major steps of Gregor Mendel’s garden pea experiments.

4. Relate the ratios that Mendel observed in his crosses to his data.

5. Describe the four major hypotheses Mendel developed.

Gamete 1 = Gamete 2 = Gamete 3 = Gamete 4 =

6. Define the terms homozygous, heterozygous, genotype, and phenotype.

7. Compare Mendel’s two laws of heredity.

Objectives Vocabulary• Predict the results of monohybrid genetic

crosses by using Punnett squares.• Apply a test cross to determine the genotype of

an organism with a dominant phenotype.• Predict the results of monohybrid genetic

crosses by using probabilities.• Analyze a simple pedigree.

• Punnett square• Test cross• Probability• pedigree• Sex-linked gene

• To predict the genotypes and phenotypes of offspring, Mendel ___________ his crosses and analyzed ________ amounts of data

• In 1905, a mathematician named Reginald ___________ devised a _______________ method, called a punnett square, for predicting the outcome of ________________ crosses

Punnett Squares

A Punnett square is a diagram that predicts the outcome of a ______________ cross by considering all possible ___________ combinations of gametes in the cross.

Remember, the two alleles in a gene _________________ during gamete formation

The possible gametes that the female can produce are written along the _________ of the square.

The possible gametes that the male can produce are written along the _________ side of the square.

• Each box inside the square is filled in with two letters obtained by ___________ the allele along the ______ of the box with the allele along the _________ of the box.

• What process is occurring in each box when the gametes combine?

• What genotype will all of the possible offspring have?

• And the phenotype?

Practice

• Cross a “female” pea plant heterozygous for seed color and a “male” recessive for seed color…

Y = yellow and y = green

Genotype ratio?

Phenotype ratio?

• The previous crosses were for genes on _________________ chromosomes

Review…what are autosomes?

• Punnett squares can also be created for ________ chromosomes– In this case, the alleles must be linked on the _______ or _______ chromosome

• In 1910, Thomas Hunt __________ discovered sex-linked inheritance while studying _______ ________.– In fruit flies, the gene for eye color is linked on the ______ chromosome

• If R = red and r = white, determine the eye color and sex for the following genotypes– XRXR = ________________– XRXr = _________________– XrXr = __________________– XRY = __________________– XrY = __________________

• In humans, red green color blindness is a _____________ trait linked on the _____ chromosome• Determine the following phenotypes…

– XCXc = _______________– XcY = ________________– XCY = ________________

Cross a two pea plants heterozygous for height…

T = tall and t = short

Genotype ratio?

Phenotype ratio?

• Reduce the ratio 2:2

• Convert this ratio to a fraction

• What percent does the fraction represent?

– The female above does not have color blindness but ____________ the recessive allelePractice

• Cross a female who is a carrier for colorblindness with a male recessive for color blindnessC = normal vision andc = colorblindGenotype ratio?

Phenotype ratio?

Does a male offspring inherit colorbindness from his mom or dad?

Review

• What genotype(s) may a purple pea plant have?• Animal breeders, horticulturists, and others involved in breeding organisms often need to know whether an

organism with a dominant phenotype is _______________ or ________________ for a trait.• To determine the genotype, breeders cross the organism with an individual that is _____________ for the trait

in a cross called a ________ crossTest Cross

• If the unknown genotype is PP, what color flowers will all of the offspring have?• If the unknown genotype is Pp, what color flowers may the offspring have?

Outcomes of Crosses• Like Punnett squares, ______________ calculations can be used to predict the results of genetic crosses.• Probability is the _________________ that a specific event will occur.• Probability = • So if there are five marbles in a jar, and only one is red, what is the probability that you pull a red marble out of

the jar?• If you flip a coin, what is the probability that it will land heads up?• If you flip a coin again, what is the probability that it will lands head up?• So prior events do not affect the outcome of ____________ events• In the following cross…

What is the probability that twoparents will have a girl?What is the probability that their next child will be a girl?

• The outcomes predicted by Punnett squares are most often seen in ______________ populations….– Is everyone’s family made up of exactly half girls and half boys?– But a larger population, like the entire _________, is roughly 50:50 or 1:1

• To determine the probability of ____________ events or events occurring at the _________ time, the probability for each event is _________________ by eachother.

• What is the probability that a coin will land heads up four times consecutively?

Inheritance of Traits• Geneticists often prepare a ________________, a family history that shows how a trait is inherited over

_____________ generations.• Pedigrees are particularly helpful if the trait is a genetic _______________ and the family members want to

know if they are _____________ or if their _________________ might get the disorder.• By analyzing a pedigree, we can determine if a trait is ______________ or ______ - linked and ____________ or

______________

Autosomal RecessiveMost genetic disorders are caused by _____________ alleles

In this situation, an offspring with ______ recessive alleles has the trait, while each parent is a _________________

What allele combinations might have the other two offspring have?

Autosomal DominantIf a trait is dominant, an affected individual will have at least ______ parent who also has the _____________ trait

Is the female parent homozygous dominant or heterozygous?How do you know?

• What is the probability that a pea plant offspring will have purple flowers?

•• What is the probability that a pea plant

offspring will be tall?•• What is the probability that a pea plant

offspring will have purple flowers and tall height?

aa

AaAa

Sex Linked

XcY

XcY

XCXc

If a trait is sex-linked, it will appear more often in ________ than in females

What genotype must all of the females have?

What chromosome does a male parent pass onto his son?

So is the recessive X-linked gene inherited from the male or female parent?

1. Predict the results of monohybrid genetic crosses by using Punnett squares.If T = tall and t = short…

Genotype ratio?Phenotype ratio?

2. Apply a test cross to determine the genotype of an organism with a dominant phenotype.

3. Predict the results of monohybrid genetic crosses by using probabilities.

4. Analyze a simple pedigree. Does this pedigree show an autosomal dominant, autosomal recessive, or sex-linked trait?