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Essentials of Biology

1.10 Mendel’s Laws

• • • • • •

Gregor Mendel

Austrian monk Worked with pea plants in 1860 When he began his work, most acknowledged that both sexes contributed equally to a new individual.

Unable to account for presence of variations among members of a family over generations • • Mendel’s model compatible with evolution Various combinations of traits are tested by the environment.

Combinations that lead to reproductive success are the ones that are passed on.

Figure 10.1 Mendel working in his garden a.

Stem length Trait Pod shape Tall Inflated b.

Short Constricted

•

Mendel’s experimental procedure  Used garden pea,

Pisum sativa

• • Easy to cultivate, short generation time Normally self-pollinate but can be cross-pollinated by hand  Chose true-breeding varieties – offspring were like the parent plants and each other.

 Kept careful records of large number of experiments   His understanding of mathematical laws of probability helped interpret results.

Particulate theory of inheritance – based on the existence of minute particles (genes)

Figure 10.2 Garden pea anatomy and traits

stigma anther ovary Pollen grains containing sperm are produced in the anther . When pollen grains are brushed onto the stigma, sperm fertilizes eggs in the ovary .

Fertilized eggs are located in ovules, which develop into seeds.

a. Flower structure

Figure 10.2 continued

1 Cut away anthers.

2 Brush on pollen from another plant.

3 The results of cross from a parent that produces round, yellow seeds × parent that produces wrinkled yellow seeds.

b. Cross pollination

• Figure 10.3 One-trait cross

One-trait inheritance  Original parents called P generation • • First-generation offspring F 1 generation Second-generation offspring F 2 generation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

P generation P gametes TT T

tt t  Crossed tall pea plants with short pea plants • • All F1 are tall Had shortness disappeared?

F 1 generation All plants are tall.

• •

Punnett square  Shows all possible combinations of egg and sperm offspring may inherit When F 1 allowed to self-pollinate, F 2 were 3/4 tall and 1/4 short  F 1 had passed on shortness.

Figure 10.3 continued

eggs F 1 gametes T t F 2 generation T t TT Tt Tt offspring tt F 2 Phenotypic Ratio 3 tall : 1 short Key: T = tall plant t = short plant

•

Mendel reasoned 3:1 ratio only possible if  F 1 parents contained 2 separate copies of each heritable factor (1 dominant and 1 recessive).

 Factors separate when gametes form and each gamete carries only 1 copy of each factor.

 Random fusion of all possible gametes occurred at fertilization.

•

One-trait testcross  To see if the F 1 carries a recessive factor, Mendel crossed his F 1 generation tall plants with true-breeding, short plants. • He reasoned that half the offspring would be tall and half would be short.

• His hypothesis that factors segregate when gametes are formed was supported..

 Testcross • Used to determine whether or not an individual with the dominant trait has two dominant factors for a particular trait

•

One-trait testcross  If a parent with the dominant phenotype has only one dominant factor, the results among the offspring are 1:1.

 If a parent with the dominant phenotype has two dominant factors, all offspring have the dominant phenotype.

Figure 10.4 One-trait testcross

Parents Parents × × Possible genotypes Tt Tt tt tt Possible genotype TT Tt tt Phenotype a.

Phenotypes Phenotypic Ratio 1 tall : 1 short b.

All tall plants

• •

Mendel’s first law of inheritance – law of segregation  Cornerstone of his particulate theory of inheritance The law of segregation states the following:  Each individual has two factors for each trait.

 The factors segregate (separate) during the formation of the gametes.

 Each gamete contains only one factor from each pair of factors.

 Fertilization gives each new individual two factors for each trait.

•

The modern genetics view  Scientists note parallel between Mendel’s particulate factors and chromosomes.

 Chromosomal theory of inheritance • Chromosomes are carriers of genetic information.

  Traits are controlled by discrete genes that occur on homologous pairs of chromosomes at a gene locus.

• Each homologue holds one copy of each gene pair.

Meiosis explains Mendel’s law of segregation and why only one gene for each trait is in a gamete.

• When fertilization occurs, the resulting offspring again have two genes for each trait, one from each parent.

• • • •

Alleles – alternative forms of a gene Dominant allele masks the expression of the recessive allele.

For the most part, an individual’s traits are determined by the alleles inherited.

Alleles occur on homologous chromosomes at a particular location called the gene locus.

Figure 10.5 Homologous chromosomes

sister chromatids G g R G G R R g g r S s S S s s t T alleles of a gene at a gene locus a. Various alleles are located at specific loci.

t t T T b. Duplicated chromosomes show that sister chromatids have identical alleles.

•

Genotype versus phenotype   Genotype – alleles individual receives at fertilization • • Homozygous – 2 identical alleles  Homozygous dominant  Homozygous recessive Heterozygous – 2 different alleles Phenotype – physical appearance of individual • Mostly determined by genotype Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Allele A Allele B Allele C Additive effect of dominant alleles on phenotype

•

Two-trait inheritance  Mendel crossed tall plants with green pods (

TTGG

) with short plants with yellow pods (

ttgg

 F 1 plants showed both dominant characteristics – tall and green pods  2 possible results for F 2 • If the dominant factors always go into gametes together, F 2  will have only 2 phenotypes.

Tall plants with green pods  Short plants with yellow pods • If four factors segregate into gametes independently, 4 phenotypes would result.

Figure 10.6 Two-trait cross done by Mendel

P generation × ttgg TTGG tg P gametes TG F 1 generation All plants are tall with green pods.

TtGg

Figure 10.6 continued

eggs F 1 gametes TG Tg tG tg TG TTGG TTGg TtGG TtGg Tg TTGg TTgg TtGg Ttgg F 2 generation tG TtGG TtGg ttGG ttGg tg F 2 Phenotypic Ratio 9 tall plant, green pod 3 tall plant, yellow pod 3 short plant, green pod 1 short plant, yellow pod TtGg Ttgg offspring ttGg ttgg Key: T = tall plant t = short plant G = green pod g = yellow pod

• • •

Based on the results, Mendel formulated his second law of heredity.

Law of independent assortment  Each pair of factors segregates (assorts) independently of the other pairs.

 All possible combinations of factors can occur in the gametes.

When all possible sperm have an opportunity to fertilize all possible eggs, the expected phenotypic results of a two trait cross are always 9:3:3:1.

•

Two-trait testcross  Fruit fly

Drosophila melanogaster

• Used in genetics research   Wild-type fly has long wings and gray body.

• Some mutants have vestigial wings and ebony bodies.

•

=long,

=short,

= gray,

=ebony Can’t determine genotype of long-winged gray-bodies fly (

_) • Cross with short-winged ebony-bodied fly (

llgg

)

•

In this example, 1:1:1:1 ratio of offspring indicates

_ fly was

LlGg

(dihybrid).

Figure 10.7 Two-trait testcross

P generation LlGg LG eggs lg LlGg llgg Lg Llgg F 1 generation lG llGg lg F 1 Phenotypic Ratio 1 long wings, gray body 1 long wings, black body 1 short wings, gray body 1 short wings, black body llgg offspring Key: L = long wings l = short wings G = gray body g = black body

•

Mendel’s laws and probability  Punnet square assumes • Each gamete contains one allele for each trait.

 Law of segregation • Collectively the gametes have all possible combinations of alleles.

 Law of independent assortment • Male and female gametes combine at random.

 Use rules of probability to calculate expected phenotype ratios.

 Rule of multiplication - chance of two (or more) independent events occurring together is the product of their chances of occurring separately.

• Coin flips – odd of getting tail is 1/2, odds of getting tails when you flip 2 coins 1/2 x 1/2= 1/4.

•

Mendel’s laws and meiosis  Gene for earlobes and hairline on different chromosomes 

Figure 10.8 Mendel’s laws and meiosis

Dominant Recessive

Gametes have all

Unattached earlobes: EE or Ee

possible combination of alleles.

Attached earlobes: ee Widow’s peak: WW or Ww Straight hairline: ww

(earlobes, both): © The McGraw-Hill Companies, Inc./John Thoeming, photographer; (widow's peak): © SuperStock; (straight): © Michal Grecco/Stock Boston

Figure 10.8 continued (potential gametes produced by a person who is EeWw)

Key: W = widow’s peak w = straight hairline E = unattached earlobes e = attached earlobes one pair e E W w one pair Parent cell has two pairs of homologues.

either or Meiosis I E E e e E E e e Homologues can align either way during metaphase I.

W W w w w w W W Meiosis II E E W W e e w w E E w w e e W W W E W E EW w e w e ew w E w E Ew W e W e eW All possible combinations of chromosomes and alleles result.

10.2 Beyond Mendel’s Laws

•

Incomplete dominance   Heterozygote has intermediate phenotype.

Four o’clock flowers • • Red, pink and white NOT blending inheritance – pink flowers can have red, white or pink offspring.

 Human wavy hair is intermediate between curly and straight hair.

Figure 10.9 Incomplete dominance in four o’clocks

C R C R C R C W C W C W

•

Multiple-allele traits  ABO blood group inheritance has 3 alleles.

• • •

i I A I B

= A antigen on red blood cells = B antigen on red blood cells = neither A or B antigen on red blood cells  Each person has only 2 of the 3 alleles.

  Both

I A

and

I B

are dominant to

i I A

and

I B

are codominant – both will be expressed equally in the heterozygote.

• • • •

Type A =

I A I A

I A i

Type B =

I B I B

I B i

Type AB =

I A I B

Type O =

Figure 10.10 Inheritance of ABO blood type

Parents I B i I A i I B I A I A eggs i l B I B i i I A i ii Phenotypic Ratio 1 : 1 : 1 : 1 offspring Key: Blood type A Blood type B Blood type AB Blood type O

•

Polygenic inheritance  Trait is governed by 2 or more sets of alleles.

   Each dominant allele has a quantitative effect on phenotype and effects are additive.

Result in continuous variation – bell-shaped curve Multifactorial traits – polygenic traits subject to environmental effects • Cleft lip, diabetes, schizophrenia, allergies, cancer • Due to combined action of many genes plus environmental influences

Figure 10.11 Height in humans, a polygenic trait

most are this height 62 short few 64 few 66 68 70 72 74 tall Height in Inches

Courtesy University of Connecticut, Peter Morenus, photographer

•

Environment and the phenotype  Relative importance of each can vary.

 Temperature can effect coat color.

• Rabbits homozygous for

have black fur where the skin temperature is low.

• Enzyme encoded by gene is active only at low temperatures.

Figure 10.12 Coat color in Himalayan rabbits

•

Pleiotropy  Single genes have more than one effect.

 Marfan syndrome is due to production of abnormal connective tissue.

Figure 10.13 Marfan syndrome, multiple effects of a single human gene

Connective tissue defects Skeleton Chest wall deformities Long, thin fingers, arms, legs Scoliosis (curvature of the spine) Flat feet Long, narrow face Loose joints Heart and blood vessels Mitral valve prolapse Enlargement of aorta Eyes Lens dislocation Severe nearsightedness Aneurysm Aortic wall tear*

Lungs Skin Collapsed lungs* Stretch marks in skin Recurrent hernias Dural ectasia: stretching of the membrane that holds spinal fluid

10.3 Sex-linked Inheritance

• • • •

Females are XX.

 All eggs contain 1 X.

Males are XY.

 Sperm contain either an X or a Y.

Y carries SRY gene – determines maleness.

X is much larger and carries more genes.

 X-linked – gene on X chromosome

Figure 10.14 Inheritance of gender in human beings

44 autosomes + XX egg 22 + X 44 autosomes + XY sperm 22 + X 22 + X 44 + XX

44 + XY

•

X-linked alleles  Fruit flies have same sex chromosome pattern as humans.

 When red-eyed female mated white mutant white-eyed male, all offspring were red-eyed.

 In the F 2 , the 3:1 ratio was found but all of the white-eyed flies were males.

  Y chromosome does not carry alleles for X linked traits.

Males always receive X from female parent, Y from male parent.

 Carrier – female who carries an X-linked trait but does not express it.

P generation Figure 10.15 x-linked inheritance X r Y X R X R P gametes X r Y X R F 1 generation X R Y X R eggs X r X R X r F 1 gametes X R F 2 generation X R X R X R X r Y F 2 Phenotypic Ratio females: all red-eyed males: 1 red-eyed 1 white-eyed X R Y offspring X r Y Key: X R X r = red eyes = white eyes

10.4 Inheritance of Linked Genes

• •

Some fruit fly crosses violated the law of independent assortment.

 Offspring simply resembled one of the parents.

2 traits on same chromosome – gene linkage

•

2 traits on same chromosome do NOT segregate independently.

•

Recombination between linked genes  Linked alleles stay together – heterozygote forms only 2 types of gametes, produces offspring only with 2 phenotypes.

Figure 10.16 Linked alleles and crossing-over

sister chromatids G R G R g r g r G R G g R r g r tetrad alleles are linked a. Linked alleles usually stay together G G g g R R r r resulting daughter chromosomes

•

Occasionally crossing-over produces new combinations.

  Nonsister chromatids exchange genes.

Recombinant gametes have a new combination of alleles.

Figure 10.16 continued

nonsister chromatids G R G R g r g r G R g r G g R R G r g r tetrad linked alleles sometimes cross-over resulting daughter chromosomes b. Crossing-over results in recombination of alleles

•

Distance between genes  The closer 2 genes are on a chromosome, the less likely they are to cross-over.

 You can use the percentage of recombinant phenotypes to determine the distance between genes.

  1% crossing-over = 1 map unit.

In a black-body and purple-eye cross, 6% of offspring are recombinant = genes are 6 map units apart.

 Results can make a chromosome map.

P generation Offspring GgRr Predicted ggrr Observed Figure 10.17 Linked alleles do not assort independently 25% GgRr 25% ggrr 25% Ggrr 25% ggRr F 1 Phenotypic Ratio 1 gray body, red eyes 1 black body, purple eyes 1 gray body, purple eyes 1 black body, red eyes 47% 47% 3% 3% Key: G = gray body g = black body R = red eyes r = purple eyes

Figure 10.18 Mapping chromosomes black body

purple eyes vestigial wings 6 map units 12.5 map units 18.5 map units

Slide 1

Transcript Slide 1

Essentials of Biology

1.10 Mendel’s Laws

10.2 Beyond Mendel’s Laws

10.3 Sex-linked Inheritance

10.4 Inheritance of Linked Genes

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