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Section A: Gregor Mendel’s DiscoveriesSection A: Gregor Mendel’s DiscoveriesSection A: Gregor Mendel’s DiscoveriesSection A: Gregor Mendel’s Discoveries

CHAPTER 14 CHAPTER 14 MENDEL AND THE GENE IDEAMENDEL AND THE GENE IDEA

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Mendel brought an experimental and quantitative approach Mendel brought an experimental and quantitative approach to geneticsto genetics

• Around 1857, Mendel began breeding Around 1857, Mendel began breeding garden peas to study garden peas to study inheritanceinheritance وراثةوراثة. . Because they are available in many Because they are available in many varieties with distinct heritable varieties with distinct heritable توارثتوارثمم characterscharacters صفاتصفات with different traits with different traits ((genesgenes).).

• Each pea plant has male (Each pea plant has male (stamensstamens) ) and female (and female (carpalcarpal) sexual organs.) sexual organs.

• In nature, pea plants typically In nature, pea plants typically self-fertilize self-fertilize ذاتى fertilizing ova , fertilizing ova ,تلقيحwith their own pollens.with their own pollens.

• However, Mendel could also move However, Mendel could also move pollens pollens اللقاح from one plant to from one plant to حبوبanother to cross-pollinate another to cross-pollinate لقح ـ لقحي ـ يplants.plants.

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• In a breeding experiment, Mendel would In a breeding experiment, Mendel would cross-pollinatecross-pollinate خلطى خلطى تلقيح -true-, true ,متباينينمتباينين two contrasting ( two contrasting (ه!جنه!جن hybridizehybridize)) تلقيح

breeding pea varieties breeding pea varieties أنواعأنواع..– The true-breeding parents are the The true-breeding parents are the P generationP generation and their hybrid and their hybrid

offspring offspring هجنالم المهجن النسل ..are the are the FF11 generation generation النسل

• Mendel would then allow the Mendel would then allow the FF11 hybrids to self-pollinate hybrids to self-pollinate

to produce an to produce an FF22 generation. generation.

• It was mainly Mendel’s It was mainly Mendel’s quantitative analysisquantitative analysis كمى كمى تحليل of of تحليلFF22 plants that revealed the two fundamental lows of plants that revealed the two fundamental lows of

heredity:heredity:

A)- A)- The law of segregation. The law of segregation. B)-B)- The law of independent The law of independent assortment. assortment.

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• The The FF11 hybrids from a cross hybrids from a cross تلقيحتلقيح between between purple-floweredpurple-flowered and and white-white-floweredflowered pea plants would have pea plants would have pale pale purplepurple flowers flowers باهت ..بنفسجى

• Instead, Instead, ولكنولكن the the FF11 hybrids all have hybrids all have

purplepurple flowers, just a purple like their flowers, just a purple like their parents.parents.

A)- Law of segregation:A)- Law of segregation: the two alleles the two alleles چـينات for a character for a character are isolated into separate gametesare isolated into separate gametes

Fig. 14.2, page 249

• This cross produced a This cross produced a 33 purple to purple to 11 white ratio of white ratio of traits in the traits in the FF22 offspring, offspring,

• Mendel reasoned that the heritable Mendel reasoned that the heritable factor for white factor for white flowers was present flowers was present in the F in the F11 plants, but it did not plants, but it did not

affect affect flower color.flower color.

• Thus, purple flower is a Thus, purple flower is a dominantdominant color color (( سائدة سائدة صفة and white flower is a and white flower is a ( (صفةrecessiverecessive one one(( متنحية متنحية صفة . . ( (صفة

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• Mendel found similar Mendel found similar 3 to 1 ratios3 to 1 ratios of two traits among of two traits among

FF22 offspring when he conducted crosses for six other offspring when he conducted crosses for six other

characters, each represented by two different varieties characters, each represented by two different varieties

مختلفتين مختلفتين صفتين ..صفتين

• For example, when Mendel crossed two true-breeding For example, when Mendel crossed two true-breeding

varieties, one of which produced varieties, one of which produced round seeds round seeds بذوربذور

بذوربذور the other of which produced , the other of which produced wrinkled seeds wrinkled seeds ,مستديرةمستديرة

all the , all the FF11 offspring had round seeds, but among offspring had round seeds, but among ,مج!عدةمج!عدة

the the FF22 plants, 75% of the seeds were round and 25% plants, 75% of the seeds were round and 25%

were wrinkled (were wrinkled (see second low in the next lecture). ).

6Table 14.1, Page 250

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• Mendel developed a hypothesis Mendel developed a hypothesis إفتراض to explain these results that to explain these results that consisted of four related ideas.consisted of four related ideas.

1. Alternative version of genes (different alleles)1. Alternative version of genes (different alleles) account for account for variations in inherited characters. variations in inherited characters.– Different alleles vary somewhat in Different alleles vary somewhat in

the sequence of nucleotides at the sequence of nucleotides at the specific locus the specific locus موضع of a gene.of a gene.

Fig. 14.3, Page 249

2. For each character, an organism2. For each character, an organism inherits inherits يرث two alleles, one fromtwo alleles, one from each parent. each parent.– These homologous loci These homologous loci على على م!وق@عه م!وق@عه

may be differmay be differ الكروموسومالكروموسوم

– In the flower-color example, the FIn the flower-color example, the F11 plants plants

inherited a inherited a purple-flower allelepurple-flower allele from one from one parent and a parent and a white-flower allelewhite-flower allele from the from the other.other.

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3. If two alleles differ, then, 3. If two alleles differ, then, thethe dominant allele dominant allele is fully expressed in is fully expressed in the organism’s appearance and the the organism’s appearance and the recessive allelerecessive allele has no effect has no effect

on the organism’s appearance.on the organism’s appearance.

4. The two alleles for each character segregate (separate) 4. The two alleles for each character segregate (separate) ينفصلينفصل during gamete production.during gamete production.

– This segregation of alleles because of the distribution of homologous This segregation of alleles because of the distribution of homologous chromosomes to different gametes in meiosis.chromosomes to different gametes in meiosis.

– If an organism has identical allele for a particular character, then that If an organism has identical allele for a particular character, then that allele exists as a single copy in all gametes.allele exists as a single copy in all gametes.

– If different alleles are present, then 50% of the gametes will receive one If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other.allele and 50% will receive the other.

• The separation of alleles into separate gametes The separation of alleles into separate gametes is summarized as Mendel’s law of segregation.is summarized as Mendel’s law of segregation.

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SummarySummary

The two alleles (genes) for a character are separated (segregated) into separate gametes and aggregated again by fertilization.

Mendelian inheritance reflects rules of probability for the behaviour of genes.

For each character, an organism inherit two allelesalleles (one from each parent).

If the two alleles differ, one of them will be DominantDominant, and the other is RecessiveRecessive.

Fig. 14.3, Page 247

Red colour Red colour gene (gene (alleleallele))

White colour gene (alleleallele)

Homologous Homologous chromosomeschromosomes

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Fig. 14.4

• A Punnett square analysis of the A Punnett square analysis of the flower-color example demonstrates flower-color example demonstrates Mendel’s model.Mendel’s model.

• Mendel’s model accounts for the Mendel’s model accounts for the 3:1 3:1 ratio in the ratio in the FF22 generationgeneration

• Mendel’s law of segregation accounts Mendel’s law of segregation accounts for the for the 3:1 ratio3:1 ratio in the in the FF22 generation. generation.

• The The FF11 hybrids will produce two hybrids will produce two

classes of gametes, half with the classes of gametes, half with the purple-flower allele and half with the purple-flower allele and half with the white-flower allele.white-flower allele.

• During self-pollination, the gametes During self-pollination, the gametes of these two classes unite randomly.of these two classes unite randomly.

• This can produce four equally likely This can produce four equally likely combinations of sperm and ovum. combinations of sperm and ovum.

• A Punnett square predicts the results A Punnett square predicts the results of a genetic cross between individuals of a genetic cross between individuals of known genotypeof known genotype الـ الـ الطرز ينىينىچچالطرز ..

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PPppPPpp

P p P p

PPPPPPPP PPppPPpp PPppPPpp pppp

PPPP pp

100% 100% PurplePurple

F2 generation

3 3 PurplePurple : 1 White : 1 White

F1 generation

Recessive Recessive alleleallele

Dominant Dominant alleleallele

PPPPPPPP ppppXX PPppPPpp PPppPPppXXPea plantPea plant

HomozygousHomozygous HeterozygousHeterozygous

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GenotypeGenotype(Genetic make up)

PPPP

PhenotypePhenotype(Colour)(Colour)

PPPPPPPP ppppppppHomozygousHomozygous

PPppPPpp HeterozygousHeterozygous

An organism having a pair of An organism having a pair of identical allelesidentical alleles

An organism having a pair of two An organism having a pair of two different allelesdifferent alleles

PhenotypePhenotype: : Is t Is the organism’s appearance.he organism’s appearance.

GenotypeGenotype: : Is t Is the organism’s genetic makeup.he organism’s genetic makeup.

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• Dominant character (allele) Dominant character (allele) السائدة الصفة

Is fully expressed in the organism’s appearance.Is fully expressed in the organism’s appearance.

• Recessive characterRecessive character (allele) (allele) تنحيةالم الصفة

Has no noticeable effect Has no noticeable effect ملحوظ غير ملحوظ تأثير غير on the organism’s on the organism’s تأثيرappearance.appearance.

• HomozygousHomozygous الجينات الجينات م�تماثل م�تماثل

An organism with two An organism with two identicalidentical alleles for a character. alleles for a character.

• HeterozygousHeterozygous الجينات الجينات م�ختلف م�ختلف

An organism with two An organism with two differentdifferent alleles for a character. alleles for a character.

• KaryotypeKaryotype الكروموسومى الكروموسومى الطرز الطرز

The display of an organism’s chromosomal patternThe display of an organism’s chromosomal pattern • PhenotypePhenotype المظهرى المظهرى الطرز الطرز

A description of an organism’s traits (feature A description of an organism’s traits (feature مظهرمظهر))..

• GenotypeGenotype الجينى الجينى الطرز الطرز

A description of an organism’s genetic makeup.A description of an organism’s genetic makeup.

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• For flower color in peas, both For flower color in peas, both PPPP and and PPpp plants have plants have the same phenotype (the same phenotype (purplepurple) but different genotypes ) but different genotypes (homozygous and heterozygous).(homozygous and heterozygous).

• The only way to The only way to produce a white produce a white phenotype is to phenotype is to be be homozygoushomozygous recessive (recessive (pppp) ) for the flower-for the flower-color gene.color gene.

Fig. 14.5

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• It is not possible to predict the genotype of an It is not possible to predict the genotype of an organism with a dominant phenotype.organism with a dominant phenotype.– The organism must have one The organism must have one dominantdominant allele, but it could be allele, but it could be

homozygoushomozygous dominant or dominant or heterozygousheterozygous..

• A test cross, breeding a A test cross, breeding a homozygous recessive homozygous recessive with dominant phenotype, with dominant phenotype, but unknown genotype,but unknown genotype,can determine the identity can determine the identity of the unknown allele. of the unknown allele.

Fig. 14.6

Q:Q: What is the result of What is the result of Cross hybridization of Cross hybridization of purplepurple X X whitewhite colored colored flowers ? flowers ?