Cell Division & Basic Concepts in Mendelian Genetics

8/14/2019 Cell Division & Basic Concepts in Mendelian Genetics

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Bio 1Cell Division and Basic Concepts

in Mendelian Genetics

SD Jacinto

Institute of Biology, UP Diliman


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Two types of cell division

Mitosis-equational division; the daughtercells produced are identical to the parentcell in terms of chromosomal content

Takes place in all cells in the body (somatic orgerm cell line) at one time or another

Meiosis- reductional division; the daughtercells have half the number of

chromosomes as the parent cellTakes place in germ cell line when they mature

to produce gametes


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http://genetics.gsk.com/chromosomes.htmhttp://www.genomenewsnetwork.org/resources/whats_a_genome/Chp1_2_2.shtml


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History of Genetics

In mid 1800's, most scientists believed that

inheritance involved ablending

of traitsbetween the parents (including Darwin!) -

i.e. a tall crossed with a short pea would

give peas intermediate in height.

www.science.siu.edu/.../ Meiosis&Genetics.html
http://www.science.siu.edu/plant-biology/PLB117/Nickrent.Lecs/Meiosis&Genetics.htmlhttp://www.science.siu.edu/plant-biology/PLB117/Nickrent.Lecs/Meiosis&Genetics.html


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Gregor Mendel, a monk, born inCzechoslovakia who later attended theUniversity of Vienna in Austria, worked withgarden peas in 1860's.

He was experienced with crossing plants andanimals to obtain desired traits. This process,called Artificial Selection, has given rise to the

tremendous variety seen in food plants, such asbeans. Mendel was the first to report the lawsgoverning inheritance of these traits.


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Mendels experiments


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a. choice of peas was good: easy to grow and cross

b. peas were cultivated and bred for a long time; lots of

variations (mutations) to work with


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c. peas are self pollinatingand easy to pollinate by hand


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d. used true breeding plants, i.e. ones that showed a

constant trait when self-pollinated


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(Parental) Greenx Yellow ---> all Green (F1)

(F1) Green X Green (self) -----> 3 Green forevery 1 Yellow (F2)

Mendel got a 3:1 ratio by counting the numberof plants that were Green (428) vs. Yellow

(152). 428/152 = 2.81:1 or essentially 3:1.

f. Mendel used statistics (probability) to interpret

his results

e. Mendel followed the inheritance of each trait

individually


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Mendelian terminologies

a) Dominantalleles(traits) are those that mask theexpression of another allele (e.g. green color).Usually written in capital letter (here G, R, S, Z)- Appear all the time.

b) Recessiveallele (traits) are those that can be maskedby a dominant allele (e.g. yellow color). Usuallywritten in lower case letters (here g, r, s, z).- Disappear and reappear .

a) and b) referred to as Mendel's principle ofdominance.

Mendel's results contradicted the popular theory of theday that traits blended.


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Mendel's Law of Segregation

"Each organism contains two factors for each trait which

separate during gamete formation so that each gametecontains only one factor.

When fertilization occurs, the new organism will have

two factors for each trait, one from each parent".

Remember, Mendel knew virtually nothing aboutcytology (chromosomes) and meiosis. We now know

his factors represent genes that occur on homologouschromosomes

Homologous chromosomes segregate during meiosis I.


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Modern Genetic Terminology to

Explain Mendel's Results

1. Gene- units of hereditary material(DNA) present in the chromosome.

- It is a segment of DNA molecule 2. Locus- the site occupied by a gene on a

DNA molecule.

3. Alleles - two or more forms of genes. 4. Homozygous alleles- identical alleles for

a specific character.


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5. Heterozygous alleles- non-identical alleles for aspecific character.

True-breeding traits exist when the organism has two

dominant alleles (e.g. TT) or two recessive alleles (tt).These organisms are termed homozygous for that geneor those alleles.

The F1 individuals above that resulted from a cross

between tall and short pea plants have both a dominant(T) allele and a recessive (t) allele. These organisms aretermed heterozygous for that gene or those alleles.

Modern Genetic Terminology to Explain

Mendel's Results(contd)


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6. Phenotype = the outward appearance of the

organisms (i.e. the physical trait that we see, e.g.

tallness).

7. Genotype = the allelic complement of the

organism, often represented by letters, e.g. the

genotype TT is called the homozygous dominant

condition, tt is the homozygous recessive, and Tt isthe heterozygous condition.

Modern Genetic Terminology to Explain

Mendel's Results(contd)


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Determination of plant traits

When looking at a plant that shows the dominantphenotype, how does one know whether it ishomozygous or heterozygous for that trait?

The two can be distinguished by conducting aMonohybrid Test Cross, which is a cross betweenan individual with a dominant phenotype with anindividual with a recessive phenotype.

By looking at the phenotypes of the offspring (F1), onecan determine whether the parent was homozygousor heterozygous.


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Two possible outcomes:

All F1 the same as the parental dominant

phenotype- then the parent was

homozygous.

ii) If only half show the dominant phenotype

and half show the recessive phenotype,

then theparent was heterozygous.


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Dihybrid Cross

Matings of parents that differ in two independent traits.For example, one can look at round (R) vs. wrinkled(r) seeds and yellow (Y) vs. green (y) seeds.

F1 progeny will all be heterozygous (RrYy) and willshow the dominant traits of being round and yellow.

When the F1 plant is selfed, four phenotypes for itsprogeny are seen in the following ratio: 9 round-

yellow, 3 wrinkled-yellow, 3 round-green and 1wrinkled-green (9:3:3:1).


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These dihybrid crosses illustrate the

principle ofindependent assortment - i.e.

one gene pair segregates independently

from another gene pair during meiosis.


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Cell Division & Basic Concepts in Mendelian Genetics

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