ZOOL 115 Lecture Notes - Lecture 12: Shorthorn, Dihybrid Cross, Sickle-Cell Disease

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Chapter 10: Patterns of Inheritance

Concept 10.1: Meiosis, Genes, and Alleles

 Genetics is the study of how the characteristics of living organisms are inherited. Classical

genetics uses an understanding of meiosis to make predictions about the genes that will be

inherited by offspring of sexually reproducing pair of organisms.

 G . . . . . . . . = a segment of DNA that determines a characteristic. Genes are transmitted from

generation to generation through cycles of meiosis and sexual reproduction.

 L . . . . . . . . of a gene = location of a gene on the chromosome

 Alleles = alternative forms of a gene in a population, located on the same pair of homologous

chromosome [Fig 10.2]. Ex. there are 2 alleles for earlobe shape. [Fig 10.1]

Genomes and Meiosis

 Genome refers to all the genes in a cell of an organism. In a sexually reproducing organism, the

genome is Diploid (2n), when it has two copies of each gene. The genome of H . . . . . . . . . . . .

cells like sperm and egg has one copy of each gene. At fertilization, the haploid sperm unites

with haploid egg to form diploid zygote with a genome that has two copies of each gene that

may be the same or different alleles.

Concept 10.2: The Fundamentals of Genetics

 Solving genetics problems relies on answering the following three questions:

- What alleles do the parents have?

- What alleles are present in the gametes that the parents produce?

- What is the likelihood that gametes with specific combinations of alleles will fertilize

each other?

 Genotype = listig of the type of alleles preset i a orgais’s cells. Upper ad lower case

letters are typically used to represent genotype (E or e)

 Phenotype = outward expression of the genotype; observable features of characteristics

(earlobe shape).

 The DOMINANT ALLELE masks the effect of the other allele when they are both present.

It is always expressed.

Upper case letters are used for dominant alleles.

 The RECESSIVE ALLELE is masked by the dominant allele.

It is only expressed in individuals homozygous for that trait.

Lower case letters are used for recessive alleles.

 The term recessive does not lower the value of the allele; it only describes how it is expressed

when present with a dominant allele.

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Predicting Gametes from Meiosis:

 An individual is Homozygous for a trait if two identical alleles (EE or ee) are present.

Heterozygous for a trait if he has two different allelic forms (Ee) of a gene

 Gene Key: Gene or Condition: earlobe type

Allele Symbols

Possible Genotypes

Phenotype

Possible Gametes

E= Free Earlobe

EE- homozygous

Free Earlobes

All gametes have E

Ee-

Earlobes

½ of gametes have E

and ½ have e

e= attached earlobes

ee-

Attached earlobes

All gametes have e

 Law of Segregation: The alleles that control a trait in a diploid organism are on different

chromosomes and separate into different gametes during gamete formation and retain their

individuality.

 Fertilization: - union of two haploid sex cells to form a diploid zygote

 A Genetic cross is a planned breeding between two organisms.

 A Punnett Square is used to show the possible offspring of a genetic cross.

 In a Single factor cross, a single characteristic is followed from one generation to the next.

 A Monohybrid cross is a cross between two individuals heterozygous for a given trait. A double

factor cross is a genetic study of two traits from parental to offspring generations at the same

time. The outcome of a cross is determined by general trends; and the Punnett Square is used to

predict the ratios of the genotypes and phenotypes for the trait being followed.

Example: Monohybrid cross of two parents heterozygous for the ear lobe shape

Cross: Ee x Ee

Possible Gametes: 50% E and 50% e 50% E and 50% e

Punnett square:

The ratio of the genotypes is 25% EE: 50% Ee: 25% ee; or 1:2:1

The ratio of the phenotypes is 75% Free earlobes: 25% Attached earlobes

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Concept 10.3: Probability vs Possibility

 Probability is the mathematical chance that an event will occur, expressed as a percentage or

fraction.

Ex. The probability of getting 4 when you roll a dice is 1/6 or 16.67%

 The probability of two or more events occurring together is the product of their individual

probabilities.

 Ex. The probability that you will get two 4’s whe you throw a pair of dice is:

1/6 x 1/6 = 1/36 or 0.0278 or 2.78%

 In a Genetics problem, the frequency with which alleles are present in gametes determines the

likelihood that a couple will have children with a characteristic. The genotype of each parent

determines what kinds of gametes he/she will produce. A Punnett square is used to show the

fertilization events.

Concept 10.4: The first Geneticist - Gregor Mendel is credited for the systematic work that led to the

current knowledge of patterns of inheritance. Mendel did not know about genes and DNA.

 Mendelian Genetics is the study of the patterns of inheritance according to the laws of Mendel.

Mendel performed crosses of pea plants with distinct (either/or) characteristics [Table 10.1] and

kept record of the results through generations. His results led him to conclusions that have since

been shown to be accurate. Some of them, however, have been modified based on current

knowledge of genes and how they are inherited. Mendel developed the idea of dominant and

recessive.

Genetic principles of Mendel:

1. Organisms have two pieces of genetic information (now called alleles) for each characteristic.

2. The alleles can be different. Medel’s Law of Doiace: When two different alleles exist for

a trait, the dominant allele masks the expression of the recessive allele.

3. Fertilization of gametes is random.

4. Medel’s Law of Segregation: The alleles that control a trait separate into different gametes

during gamete formation and retain their individuality.

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Document Summary

Genetics is the study of how the characteristics of living organisms are inherited. Classical genetics uses an understanding of meiosis to make predictions about the genes that will be inherited by offspring of sexually reproducing pair of organisms. = a segment of dna that determines a characteristic. Genes are transmitted from generation to generation through cycles of meiosis and sexual reproduction. L . of a gene = location of a gene on the chromosome. Alleles = alternative forms of a gene in a population, located on the same pair of homologous chromosome [fig 10. 2]. Ex. there are 2 alleles for earlobe shape. Genome refers to all the genes in a cell of an organism. In a sexually reproducing organism, the genome is diploid (2n), when it has two copies of each gene. The genome of h . cells like sperm and egg has one copy of each gene.

Related Questions

Ploidy refers to the number of complete sets of chromosomes (genes) present in a cell. The number of chromosomes in a complete set is a fixed characteristic of a species. Humans are diploid with two sets of 23 chromosomes. Fruit flies are diploid with two sets of 4 chromosomes. Sexually reproducing organisms are diploid, each somatic cell of an organism has two complete sets ("di" is a prefix indicating 2.). To complete a life cycle and reproduce new members of a species, mating is required. Mating brings a donation from a male and a donation from a female together. However, if the male and female cells remain diploid, the fertilization will result in a tetraploid zygote. In general, these are not viable. A mechanism is needed to form special haploid cells, gametes. The diploid set is split precisely into two complete haploid sets. That mechanism is meiosis.

The complete set of chromosomes of a species is also the complete set of genes of a species, the genome. (Yes, there is a minor contribution from mitochondria and chloroplasts. Inheritance of this information is "cytoplasmic".)

Please answer the following questions:

1. How many chromosomes in a brain cell of a fruit fly? How many chromosomes are in a liver cell of a human? Explain both answers.

2. How many chromosomes are in a fruit fly unfertilized egg? How many chromosomes are in a human egg? Explain both answers.

3. Why is meiosis necessary for sexually reproducing organisms? How many chromosomes are in a human zygote? Relate the two answers.

4. What is a gene allele? Suppose there is a gene called "bingo" and it is present once in a complete haploid set. How many alleles of "bingo" does a diploid individual have? How many copies or versions of the gene "bingo" are present in a diploid individual?

5. Modern DNA sequencing clearly identifies alleles by showing the different DNA sequence of each allele. These alleles may be termed structural alleles (I coined the term for this problem.). Classical genetics only dealt with phenotypes or visible traits. As a result, alleles of genes were identified by functionality, usually normal or non-functional. Using your knowledge of gene structure and gene expression, postulate a relationship between structural alleles and functional alleles. (include relative amounts; more, less, or equal).

If an organismâs diploid chromosome number is 18, how many different possible combinations of homologous chromosomes lining up during meiosis exist for the eggs or sperm produced by that organism?

	A.	512
	B.	9
	C.	18
	D.	128
	E.	36

At the end of metaphase I, _______________ separate.

	A.	sister chromatids
	B.	germ cells
	C.	homologous chromosomes
	D.	haploid chromatids
	E.	centrioles

Mendel observed that dominant traits

	A.	are seen in all of the F1 hybrid pea plants in his experiments.
	B.	are expressed in all plants.
	C.	were absent in the F1 generation of pea plants that he used in his experiments.
	D.	were the only traits seen in the F2 generation of pea plants in his experiments.
	E.	are only expressed in hybrids.

Cytokinesis in plant cells differs from cytokinesis in animal cells because

	A.	there is no difference.
	B.	in plant cells, the cell plate must also divide into two parts.
	C.	the contractile protein, actin, is important only in plant cells.
	D.	plant cells have a rigid cell wall.
	E.	a contractile ring forms only in plant cells.

Sickle cell anemia is an example of what type of inheritance?

	A.	complete dominance
	B.	incomplete dominance
	C.	codominance
	D.	multiple alleles
	E.	recessive dominance

Which of the following statements is true:

	A.	The dominant allele is masked in homozygous dominant individuals.
	B.	With recessive genetic disorders, if both parents are carriers, the offspring will all be affected.
	C.	In carriers, the recessive allele causes an intermediate phenotype.
	D.	In recessive genetic disorders, the mother and/or father of an affected individual must also be affected.
	E.	With dominant genetic disorders, the mother and/or father of an affected individual must also be affected.

Skin cancers typically develop in the

	A.	upper layers of the epidermis.
	B.	lower layers of the dermis.
	C.	subcutaneous layer.
	D.	lower layers of the epidermis.
	E.	upper layers of the dermis.

Duchenne muscular dystrophy (MD) is inherited from an X-linked recessive allele. What is the probability that a son with Duchenne MD inherited this disease from his biological father?

	A.	1/2
	B.	0
	C.	1/16
	D.	1/4
	E.	1/8

The genetic makeup of a particular trait in an individual is its

	A.	genotype.
	B.	heterozygosity.
	C.	phenotype.
	D.	filial.
	E.	dominance.

What structure holds the sister chromatids to the spindle fibers?

	A.	chromatin
	B.	kinetochore
	C.	MPF
	D.	centromere
	E.	cyclin

If you view a cell in which the genetic material is beginning to be visible as separate bodies, and the nucleus has disappeared from view, you may surmise that the cells is in

	A.	telophase.
	B.	anaphase.
	C.	interphase.
	D.	metaphase.
	E.	prophase.

Gregor Mendel was successful in his analysis of the genetics of pea plants because

	A.	he examined and analyzed both the F1 and F2 generations.
	B.	he studied the parental plants to determine their differences.
	C.	he decided to only look at his results in an objective manner.
	D.	he studied a trait that had a strange inheritance pattern.
	E.	pea plants have genetics different from other organisms.

Tall corn plants (T) are dominant to dwarf plants (tt). Solid green leaves (G) are dominant to leaves with a white tip (gg). A cross between two corn plants yielded the following phenotypes: 51 tall plants with a white tip on their leaves; 43 dwarf plants with solid leaves; 48 dwarf plants with white tips on their leaves; 45 tall plants with solid leaves. What are the genotypes of the parents that produced these plants?

	A.	None of the above
	B.	TtGg x Ttgg
	C.	ttGG x TTgg
	D.	TtGg x TtGg
	E.	TtGg x ttgg

In humans, a gene that has been identified as causing a type of skin cancer is the

	A.	superwoman echidna.
	B.	mutant superman.
	C.	sonic hedgehog.
	D.	mutant mole rat.
	E.	superhero aardvark.

The segregation principle states that in sexually reproducing diploid organisms the two copies of each gene

	A.	segregate from each other during meiosis.
	B.	must always be the same allele.
	C.	separate from each other during mitosis.
	D.	will both wind up in either the sperm or egg.
	E.	move together as a unit during meiosis.

In what phases is the genetic material in the cell correctly referred to as chromatids?

	A.	metaphase and telophase
	B.	anaphase and metaphase
	C.	interphase and telophase
	D.	interphase and prophase
	E.	metaphase and prophase

Consider two traits for an organism, determined by two genes, each of which is governed by at least two alleles. In the case of a dihybrid individual, the gametes formed will be of either the parental type or the recombinant type. Recombinant type gametes are formed because of

	A.	the principle of dihybrids.
	B.	multiple alleles.
	C.	heterozygosity.
	D.	incomplete dominance.
	E.	independent assortment.

An allele is

	A.	an alternate form of a gene.
	B.	always recessive.
	C.	the main factor determining a trait.
	D.	always one of a pair.
	E.	the dominant form of a gene.

After the DNA is replicated, and it condenses in prophase, two identical rods of DNA are seen. These are

	A.	spindle fibers.
	B.	kinetochores.
	C.	chromatids.
	D.	chromatin.
	E.	centromeres.

Special cells found in the gonads that give rise to gametes upon division are called

	A.	egg cells.
	B.	somatic cells.
	C.	germ cells.
	D.	stem cells.
	E.	basal cells.

If an organismâs diploid chromosome number is 18, how many different possible combinations of homologous chromosomes lining up during meiosis exist for the eggs or sperm produced by that organism?

	A.	512
	B.	9
	C.	18
	D.	128
	E.	36

At the end of metaphase I, _______________ separate.

	A.	sister chromatids
	B.	germ cells
	C.	homologous chromosomes
	D.	haploid chromatids
	E.	centrioles

Mendel observed that dominant traits

	A.	are seen in all of the F1 hybrid pea plants in his experiments.
	B.	are expressed in all plants.
	C.	were absent in the F1 generation of pea plants that he used in his experiments.
	D.	were the only traits seen in the F2 generation of pea plants in his experiments.
	E.	are only expressed in hybrids.

Cytokinesis in plant cells differs from cytokinesis in animal cells because

	A.	there is no difference.
	B.	in plant cells, the cell plate must also divide into two parts.
	C.	the contractile protein, actin, is important only in plant cells.
	D.	plant cells have a rigid cell wall.
	E.	a contractile ring forms only in plant cells.

Sickle cell anemia is an example of what type of inheritance?

	A.	complete dominance
	B.	incomplete dominance
	C.	codominance
	D.	multiple alleles
	E.	recessive dominance

Which of the following statements is true:

	A.	The dominant allele is masked in homozygous dominant individuals.
	B.	With recessive genetic disorders, if both parents are carriers, the offspring will all be affected.
	C.	In carriers, the recessive allele causes an intermediate phenotype.
	D.	In recessive genetic disorders, the mother and/or father of an affected individual must also be affected.
	E.	With dominant genetic disorders, the mother and/or father of an affected individual must also be affected.

Skin cancers typically develop in the

	A.	upper layers of the epidermis.
	B.	lower layers of the dermis.
	C.	subcutaneous layer.
	D.	lower layers of the epidermis.
	E.	upper layers of the dermis.

Duchenne muscular dystrophy (MD) is inherited from an X-linked recessive allele. What is the probability that a son with Duchenne MD inherited this disease from his biological father?

	A.	1/2
	B.	0
	C.	1/16
	D.	1/4
	E.	1/8

The genetic makeup of a particular trait in an individual is its

	A.	genotype.
	B.	heterozygosity.
	C.	phenotype.
	D.	filial.
	E.	dominance.

What structure holds the sister chromatids to the spindle fibers?

	A.	chromatin
	B.	kinetochore
	C.	MPF
	D.	centromere
	E.	cyclin

If you view a cell in which the genetic material is beginning to be visible as separate bodies, and the nucleus has disappeared from view, you may surmise that the cells is in

	A.	telophase.
	B.	anaphase.
	C.	interphase.
	D.	metaphase.
	E.	prophase.

Gregor Mendel was successful in his analysis of the genetics of pea plants because

	A.	he examined and analyzed both the F1 and F2 generations.
	B.	he studied the parental plants to determine their differences.
	C.	he decided to only look at his results in an objective manner.
	D.	he studied a trait that had a strange inheritance pattern.
	E.	pea plants have genetics different from other organisms.

	A.	None of the above
	B.	TtGg x Ttgg
	C.	ttGG x TTgg
	D.	TtGg x TtGg
	E.	TtGg x ttgg

In humans, a gene that has been identified as causing a type of skin cancer is the

	A.	superwoman echidna.
	B.	mutant superman.
	C.	sonic hedgehog.
	D.	mutant mole rat.
	E.	superhero aardvark.

The segregation principle states that in sexually reproducing diploid organisms the two copies of each gene

	A.	segregate from each other during meiosis.
	B.	must always be the same allele.
	C.	separate from each other during mitosis.
	D.	will both wind up in either the sperm or egg.
	E.	move together as a unit during meiosis.

In what phases is the genetic material in the cell correctly referred to as chromatids?

	A.	metaphase and telophase
	B.	anaphase and metaphase
	C.	interphase and telophase
	D.	interphase and prophase
	E.	metaphase and prophase

	A.	the principle of dihybrids.
	B.	multiple alleles.
	C.	heterozygosity.
	D.	incomplete dominance.
	E.	independent assortment.

An allele is

	A.	an alternate form of a gene.
	B.	always recessive.
	C.	the main factor determining a trait.
	D.	always one of a pair.
	E.	the dominant form of a gene.

After the DNA is replicated, and it condenses in prophase, two identical rods of DNA are seen. These are

	A.	spindle fibers.
	B.	kinetochores.
	C.	chromatids.
	D.	chromatin.
	E.	centromeres.

Special cells found in the gonads that give rise to gametes upon division are called

	A.	egg cells.
	B.	somatic cells.
	C.	germ cells.
	D.	stem cells.
	E.	basal cells.

ZOOL 115 Lecture Notes - Lecture 12: Shorthorn, Dihybrid Cross, Sickle-Cell Disease

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