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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

MENDEL AND THE GENE IDEAMENDEL AND THE GENE IDEA

ObjectivesObjectives

• DistinguishDistinguish between dominant and recessive between dominant and recessive traits.traits.

• StateState two laws of heredity that were developed two laws of heredity that were developed from Mendel’s work.from Mendel’s work.

• DescribeDescribe how Mendel’s results can be how Mendel’s results can be explained by scientific knowledge of genes and explained by scientific knowledge of genes and chromosomes.chromosomes.

• DifferentiateDifferentiate between the genotype and the between the genotype and the phenotype of an organism.phenotype of an organism.

• ExplainExplain how probability is used to predict the how probability is used to predict the results of genetic crosses.results of genetic crosses.

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Mendel brought an experimental and Mendel brought an experimental and quantitative approach to geneticsquantitative approach to 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 (carpelcarpel) sexual organs.) sexual organs.

• In nature, pea plants typically In nature, pea plants typically self-fertilize self-fertilize ذات يتلقيح , fertilizing ova , fertilizing ova with 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.

Three Steps of Mendel’s ExperimentsThree Steps of Mendel’s Experiments

Mendel’s Results and ConclusionsMendel’s Results and Conclusions

Recessive and Dominant TraitsRecessive and Dominant Traits

Mendel concluded that inherited characteristics Mendel concluded that inherited characteristics are controlled by are controlled by factorsfactors that occur in pairs. that occur in pairs.

In his experiments on pea plants, one factor in a In his experiments on pea plants, one factor in a pair masked the other. The trait that masked the pair masked the other. The trait that masked the other was called the other was called the dominantdominant trait. The trait that trait. The trait that was masked was called the was masked was called the recessiverecessive trait. trait.

Recessive and Dominant TraitsRecessive and Dominant Traits

Mendel concluded that inherited characteristics Mendel concluded that inherited characteristics are controlled by are controlled by factorsfactors that occur in pairs. that occur in pairs.

In his experiments on pea plants, one factor in a In his experiments on pea plants, one factor in a pair masked the other. The trait that masked the pair masked the other. The trait that masked the other was called the other was called the dominantdominant trait. The trait that trait. The trait that was masked was called the was masked was called the recessiverecessive trait. trait.

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

breeding pea varieties breeding pea varieties أنواعأنواع..– The true-breeding parents are the The true-breeding parents are the P P ((PParental generationarental generation)) and and

their hybrid offspring their hybrid offspring هجنالم المهجن النسل . are the are the FF11 ((11stst FFilial generationilial generation) .) النسل

• Mendel would then allow the Mendel would then allow the FF11 ( (11stst FFilial generationilial generation) hybrids ) hybrids

to self-pollinate to produce an to self-pollinate to produce an FF22 generation. generation.

• It was mainly Mendel’s It was mainly Mendel’s quantitative analysisquantitative analysis كم كم تحليل ييتحليل of of FF22 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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• Mendel expected that Mendel expected that the the FF11 hybrids hybrids

from a cross from a cross تلقيحتلقيح between between purple-purple-floweredflowered and and white-floweredwhite-flowered pea pea plants would have plants would have pale purplepale purple flowers flowers

باهت يبنفسج ..

• 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: اإلنعزال the two alleles the two alleles ((قانون for a character are isolated into separate for a character are isolated into separate چـيناتgametesgametes

• 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 color color(( متنحية متنحية صفة . . ( (صفة

• Mendel found similar Mendel found similar 3 : 1 ratios3 : 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 offspring had roundround seeds, but among seeds, but among ,مج!عدةمج!عدة

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

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

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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 1. Alternative version of genes (different alleles المتقابلين چالـ ينين ) accountaccount

for variations in inherited characters.for 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.

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!وقFعه م!وقFعه

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.

The two alleles (genes) for a character are separated (segregated) into separate gametes (summarized as Mendel’s law of segregationsummarized as Mendel’s law of segregation) and aggregated again by fertilization.

Mendelian inheritance reflects rules of probability for the behaviour of genes (alleles).

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.

Red colour Red colour gene (gene (alleleallele))

White colour gene (alleleallele)

Homologous Homologous chromosomeschromosomes

Alleles segregate تنفصل because of the distribution of homologous chromosomes to different gametes in meiosis.

• 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 of known genotype genotype الـ الـ الطرز ييينينچچالطرز ..

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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

HomozygousHomozygousHeterozygousHeterozygous

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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 tIs the organism’s appearance he organism’s appearance الطرز الطرزييالمظهرالمظهر ..GenotypeGenotype: : Is t Is the organism’s genetic makeup he organism’s genetic makeup

الـچين الـچين الطرز ييالطرز ..

متماثل الجينات

متباين الجينات

• 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.

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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..

• Test crossTest cross, ,

is is breeding a breeding a homozygous recessive homozygous recessive with with dominant phenotypedominant phenotype, , but but unknown genotypeunknown genotype,,can determine the identity can determine the identity of the unknown allele. of the unknown allele.

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

The Law of The Law of SegregationSegregation The law of segregation states that a pair of factors is segregated, or

separated, during the formation of gametes.

Dominant character (alleleDominant character (allele) ) السائدة الصفة Is fully expressed in the organism’s appearance.Is fully expressed in the organism’s appearance.

Recessive character (allele)Recessive character (allele) تنحية الم� الصفة Has no noticeable effect Has no noticeable effect ملحوظ غير ملحوظ تأثير غير on the organism’s on the organism’s تأثير

appearance.appearance.

HomozygousHomozygous الجينات تماثل الجينات م� تماثل م� 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.

PhenotypePhenotype المظهر المظهر الطرز ييالطرز A description of an organism’s traits (feature A description of an organism’s traits (feature مظهرمظهر).).

GenotypeGenotype الجين الجين الطرز ييالطرز A description of an organism’s genetic makeup.A description of an organism’s genetic makeup.

DefinitionsDefinitions