Ch 14 Mendel and the Gene Idealms.ndmctsgh.edu.tw/sysdata/72/6772/doc/738fcb71cd43533f/attach/106813.pdf · 10 - Dihybrid cross二對性狀雜交 -law of independent assortment: the

Ch 14 Mendel and the Gene Idea (Mendelian Genetics)

BioA Mendel Genetics-JYCHEN-2017

陳正繹

國防醫學院生物及解剖學科

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14.0 Drawing from the Deck of Genes What genetic principles account for the passing of traits性狀 from parents

to offspring?

The “blending” hypothesis混成假說

The “particulate” hypothesis粒子假說 :discrete heritable units (genes)

Mendel documented a particulate mechanism through his experiments with garden peas

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14.1: Mendel used the scientific approach to identify two laws

of inheritance A. Mendel’s Experimental, Quantitative

Approach

1. Choice of organism (garden pea) a. Grow easily b. Relatively short life spans c. Numerous and distinct traits d. Mating of individuals can be controlled

2. Experimental approach a. Self-fertilization (自花授粉)and

cross-fertilization(異花授粉) b. Characteristics (性狀) c. True-breeding (純種) and hybrid (雜交種)

d. Hybridization or cross (雜交) e. P generation: (P for parental親代),

F1 generation (F for filial; 第一子代), and F2 generation (第二子代).

character: A heritable feature that

varies among individuals

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When F1 hybrids pea plants are allowed to self-

pollinate, which traits appear in the F2

generation

B. Mendel’s law of segregation分離律describes inheritance of a single characteristic

1. Monohybrid cross單一性狀雜交

F1: all were purple

F2: purple to white = 3:1

Mendel called a “heritable factor” is

what we now call a gene

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an egg or a sperm gets only one of the two alleles 對偶基因that are present in the somatic cells of an organism

The segregation of alleles corresponds to the distribution of homologous chromosomes同源染色體 to different gametes in meiosis

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1. Mendel’s Model Four hypotheses:

a. There are alternative forms of genes are called alleles 對偶基因account for variations in inherited characters.

b. For each characters, an organism has two alleles, one from each parent.

c. if the two alleles at a locus differ, then one (the dominant allele顯性) determines the organism’s appearance, and the other (the recessive allele隱性) has no noticeable effect on appearance

d. The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.

Law of segregation: Pairs of gene segregate during gamete formation; the fusion of gametes at fertilization pairs gene once again.

Mendel’s law of segregation

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2. Useful Genetic Vocabulary

a. Homozygous同基因型 and heterozygous異基因型. b. Phenotype外表型(physical appearance) and genotype基因型

(genetic makeup). c. Punnett square龐尼特方格; 棋盤格

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3. The Testcross檢定雜交

*Breeding the mystery individual with a homozygous recessive individual, if any offspring display the recessive phenotype, the mystery parent must be heterozygous

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- Dihybrid cross二對性狀雜交

- law of independent assortment: the alleles of a pair segregate independently of other allele pairs during gamete formation

- This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome

- Genes located near each other on the same chromosome tend to be inherited together

C. The law of independent assortment 獨立分配律is revealed by tracking two characteristics at once

Monohybrid cross & Dihybrid cross

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Q1.Match each term on the left with a statement on the right.

1)Gene A) A variant for a character 2) Allele B) A cross between an individual with an unknown genotype

and a homozygous recessive individual 3) Character 4) Trait C) Having two identical alleles for a gene 5) Dominant allele 6) Recessive allele D) Having two different alleles for a gene 7) Genotype E) An alternative version of a gene

8) Phenotype F) An organism's appearance or observable traits

9) Homozygous

10)Heterozygous G) A heritable unit that determines a character and can exist in different forms.

11)Testcross

12) Monohybrid cross H) Determines phenotype in a heterozygote

I) A heritable feature that varies among individuals

J) The genetic makeup of an individual

K) A cross between individuals heterozygous for a single character

L) Has no effect on phenotype in a heterozygote

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A. The Multiplication乘機率 and Addition Rules加法律applied to monohybrid crosses

1. Rule of multiplication乘積率 : calculates the probability of two independent events

2. Rule of addition加法律 : calculates the probability of an event that can occur in alternate ways

14.2: Probability laws 機率 govern Mendelian inheritance.

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Q2. The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four genes, what are the probabilities that F2 offspring will have the following genotypes?

A) aabbccdd

B) AaBbCcDd

C) AABBCCDD

D) AaBBccDd

E) AaBBCCdd

Q.3 What is the probability that each of the following pairs of parents will produce the indicated offspring? (Assume independent assortment of all gene pairs.) A) AABBCC × aabbcc AaBbCc

B) AABbCc × AaBbCc AAbbCC

C) AaBbCc × AaBbCc AaBbCc

D) aaBbCC × AABbcc AaBbcc

B. Solving Complex Genetics Problems with the Rules of Probability

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14.3: Inheritance patterns are often more complex than

predicted by simple Mendelian genetics

A. Extending Mendelian Genetics for a Single Gene

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

i. When alleles are not completely dominant or recessive

ii. When a gene has more than two alleles

iii. When a gene produces multiple phenotypes

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1. Degrees of Dominance

Complete dominance 完全顯性occurs when phenotypes of the heterozygote and dominant homozygote are identical

In incomplete dominance不完全顯性, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

In codominance共顯性, two dominant alleles affect the phenotype in separate, distinguishable ways. Ex. MN blood type

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a. The Relationship Between Dominance and Phenotype

* A dominant allele does not subdue a recessive allele; alleles don’t interact

* Alleles are simply variations in a gene’s nucleotide sequence

* For any character, dominance / excessiveness relationships of alleles depend on the level at which we examine the phenotype

* Tay-Sachs disease台薩氏症is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain

i. At the organismal有機體的 level, the allele is recessive

ii. At the biochemical 生化的level, the phenotype (i.e., the enzyme activity level) is incompletely dominant

iii. At the molecular level分子, the alleles are codominant

http://medicalpicturesinfo.com/tay-sachs-disease/

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b. Frequency of Dominant Alleles

1. Multiple Alleles複數對偶基因:most genes exist in populations in more than two allelic forms

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2. Pleiotropy基因多型性

* Most genes have multiple phenotypic effects, a property called pleiotropy • ex. Sickle-cell anemia鐮刀形細胞貧血症

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A. Extending Mendelian Genetics for a Single Gene

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B. Extending Mendelian Genetics for Two or More Genes

Some traits may be determined by two or more genes

1. Epistasis上位性作用；異位顯性

In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

For example, in mice and many other mammals, coat color depends on two genes

One gene determines the pigment color (with alleles B for black and b for brown)

The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair

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Quantitative characters are those that vary in the population along a continuum

Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

Skin color in humans is an example of polygenic inheritance

2. Polygenic Inheritance多基因遺傳

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Extending Mendelian Genetics for Two or More Genes

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Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

The phenotypic range is broadest for polygenic characters

Traits that depend on multiple genes combined with environmental influences are called multifactorial多因素

C. Nature and Nurture營養: The Environmental Impact 環境影響因素on Phenotype

D. A Mendelian View of Heredity and Variation * An organism’s phenotype includes its physical appearance, internal

anatomy, physiology, and behavior

* An organism’s phenotype reflects its overall genotype and unique environmental history

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* Humans are not good subjects for genetic research

i. Generation time is too long

ii. Parents produce relatively few offspring

iii. Breeding experiments are unacceptable

* However, basic Mendelian genetics endures as the foundation of human genetics

A. Pedigree Analysis族譜分析

* A pedigree is a family tree that describes the interrelationships of parents and children across generations

* Inheritance patterns of particular traits can be traced and described using pedigrees

* Pedigrees can also be used to make predictions about future offspring

* We can use the multiplication and addition rules to predict the probability of specific phenotypes

14.4: Many human traits follow Mendelian patterns of inheritance

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Pedigree Analysis:

widow’ peak a dominant or recessive trait?

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Pedigree Analysis:

taste PTC a dominant or recessive trait?

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B. Recessively Inherited Disorders * Many genetic disorders are inherited in a recessive manner, these range from

relatively mild to life-threatening

1. The Behavior of Recessive Alleles

* Recessively inherited disorders show up only in individuals homozygous for the allele

* Carriers 帶原者are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)

* Albinism 白子症is a recessive condition characterized by a lack of pigmentation in skin and hair

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* If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low

* Consanguineous matings近親通婚(i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele

* Most societies and cultures have laws or taboos禁忌against marriages between close relatives

2. Cystic Fibrosis纖維性囊腫

* Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent (4% carrier)

*The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes

*Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

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3. Sickle-Cell Disease鐮型紅血球

* Sickle-cell disease affects one out of 400 African-Americans (10% carrier)

* The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells.

* In homozygous individuals, all hemoglobin is abnormal (sickle-cell); heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms

* Symptoms include physical weakness, pain, organ damage, and even paralysis

* Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common

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* Dominant alleles that cause a lethal disease are rare and arise by mutation

* Achondroplasia軟骨發育不全is a form of dwarfism caused by a rare dominant allele

1. Huntington’s Disease

* Huntington’s disease亨丁頓舞蹈症is a degenerative disease of the nervous system

* The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

C. Dominantly Inherited Disorders

D. Multifactorial Disorders * Many diseases, such as heart disease and cancer, have both genetic and

environmental components

* No matter what our genotype, our lifestyle has a tremendous effect on phenotype

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Q4. The pedigree below traces the inheritance of alkaptonuria黑尿症, a biochemical disorder. Affected individuals, indicated here by the colored circles and squares, are unable to metabolize a substance called alkapton, which colors the urine and stains body tissues. Does alkaptonuria appear to be caused by a dominant allele or by a recessive allele? Fill in the genotypes of the individuals whose genotypes can be deduced. What genotypes are possible for each of the other individuals?

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E. Genetic Testing and Counseling諮商

*Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

1. Counseling Based on Mendelian Genetics and Probability Rules

*Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders,

2. Tests for Identifying

Carriers

*For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately

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3. Fetal Testing胎兒檢測

* In amniocentesis羊膜穿刺術, the liquid that bathes the fetus is removed and tested – sample start 15th wk

* In chorionic villus sampling (CVS) 絨膜絨毛採樣, a sample of the placenta is removed and tested -- sample start 10th wk

15th wk 10th wk

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* Fetal imaging胎兒影像: ultrasound imaging超音波allow fetal health to be assessed visually in utero.

4. Newborn screening (新生兒篩檢): Some genetic disorders can be detected at

birth by simple tests, allowing affected infants to receive immediate medical attention. Ex. Phenylketouria (PKU 苯酮尿症)

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You should now be able to: Define the following terms: true breeding, hybridization,

monohybrid cross, P generation, F1 generation, F2 generation

Distinguish between the following pairs of terms: dominant and recessive; heterozygous and homozygous; genotype and phenotype

Use a Punnett square to predict the results of a cross and to state the phenotypic and genotypic ratios of the F2 generation

Explain how phenotypic expression in the heterozygote differs with complete dominance, incomplete dominance, and codominance

Define and give examples of pleiotropy and epistasis

Explain why lethal dominant genes are much rarer than lethal recessive genes

Explain how carrier recognition, fetal testing, and newborn screening can be used in genetic screening and counseling

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Extending Mendelian Genetics for a Single Gene

Q: Degrees of Dominance

A. Complete dominance

B. incomplete dominance

C. codominance