Gregor Mendel’s DiscoveriesGregor Mendel’s Discoveries

MENDEL AND THE GENE IDEAMENDEL AND THE GENE IDEA

Lecture 8: GeneticsLecture 8: Genetics

ObjectivesObjectives

Distinguish between dominant and recessive traits.

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

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

Differentiate between the genotype and the phenotype of an organism.

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

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

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

• 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 يُـلقحيُـلقح plants.plants.

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

In a breeding experiment, Mendel would In a breeding experiment, Mendel would cross-cross-pollinatepollinate ييتلقيح خلطتلقيح خلط ((hybridizehybridize هَجنهَجن) two contrasting ) two contrasting ..أنواعأنواع true-breeding pea varieties , true-breeding pea varieties ,متباينينمتباينين The true-breeding parents are the The true-breeding parents are the P P ((PParental arental

generationgeneration)) and their hybrid offspring and their hybrid offspring النسل الُمهجنالنسل الُمهجن are the are the FF11 ((11stst FFilial generationilial generation) .) .

Mendel would then allow the Mendel would then allow the FF11 ( (11stst FFilial generationilial generation) ) hybrids to self-pollinate to produce an hybrids to self-pollinate to produce an FF22 generation.generation.

It was mainly Mendel’s It was mainly Mendel’s quantitative analysisquantitative analysis تحليل تحليلييكمكم of of FF22 plants that revealed the two plants that revealed the two

fundamental lows of heredity:fundamental lows of heredity: A)- A)- The law of segregation. The law of segregation. B)-B)- The law of independent assortment. The law of independent assortment.

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

Mendel expected that Mendel expected that the the FF11 hybrids from a cross hybrids from a cross تلقيحتلقيح between between purple-floweredpurple-flowered and and white-floweredwhite-flowered pea plants would pea plants would have have pale purplepale purple flowers flowers يبنفسج ..باهت

Instead, Instead, ولكنولكن the the FF11 hybrids all hybrids all have have purplepurple flowers, just flowers, just a purple like their parents.a purple like their parents.

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• 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 of two traits among among FF22 offspring when he conducted crosses offspring when he conducted crosses for six other characters, each represented by for six other characters, each represented by two different varieties two different varieties صفتين مختلفتينصفتين مختلفتين..

For example, when Mendel crossed two true-For example, when Mendel crossed two true-breeding varieties, one of which produced breeding varieties, one of which produced round seeds round seeds ُمستديرةُمستديرة بذوربذور , the other of which , the other of which produced produced wrinkled seeds wrinkled seeds ُمَجعدةُمَجعدة بذوربذور , all the , all the FF11

offspring had offspring had roundround seeds, but among the seeds, but among 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). ).

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

1. Alternative version of genes 1. Alternative version of genes (different alleles ينين چالـaccount account (المتقابلين

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 َموقِعُه َموقِعُه

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 A Punnett square predicts the results of a genetic cross results of a genetic cross between individuals of known between individuals of known genotype genotype ييينينچچالطرز الـالطرز الـ ..

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PPpp

P p P p

PPPP PPpp PPpp pp

PP p

100% 100% PurplePurple

F2 generation

3 3 PurplePurple : 1 White : 1 White

F1 generation

Recessive Recessive alleleallele

Dominant Dominant alleleallele

PPPP ppXX PPpp PPppXXPea plantPea plant

HomozygousHomozygousHeterozygousHeterozygous

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GenotypeGenotype(Genetic make up)

PPPP

PhenotypePhenotype(Colour)(Colour)

PPPP ppppHomozygousHomozygous

PPpp 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 الطرز

ييالمظهرالمظهر ..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 the same phenotype (plants have the same phenotype (purplepurple) but ) but different genotypes (homozygous and different genotypes (homozygous and heterozygous).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.

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 allele, but it

could be 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.

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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 ?