GENERAL BIOLOGY 1 Course code: BLY 101

Credit units: 3 Instructor: Olukemi Aromolaran (PhD) Office: Room 36, College of Agriculture, Engineering and Science Office hours: 8am – 5pm

Recommended text

• Taylor, D.J., Green, N.P.O and Stout, G.W. (2010). Biological Sciences. Cambridge University Press, 10th Edition, South Africa.

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INTRODUCTORY GENETICS

Introductory Genetics

Genetics is the study of genes, genetic variation and heredity.

For a long time, people have known that many traits can be transmitted from one generation to another. This knowledge have been used in breeding more desirable fruits, plants and animals.

However, scientists could not explain why some traits in the parent were not physically observed in the off-springs or why some other traits disappear for a time in the lineage until the mid of 1900s, when Gregor Mendel, an Austrian monk performed some experiments using garden peas – Pisum sativum.

Mendelian genetics

Around 1866, the Austrian monk, Gregor Mendel demonstrated that characters are passed from parent to their offspring as discrete units. These units were later called genes by a Danish botanist, Wilhelm Johannsen in 1909 and Thomas Hunt Morgan, an American scientist showed that these genes are carried on the chromosomes.

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Garden peas are self pollinating, therefore the offspring has the same characteristics has the single parent.

Pollination – carrying male gametes to the female part of a plant for fertilization to occur in order to produce a new cell.

Mendel was able to have true/pure breed offspring during his experiment because pea plants are self pollinating.

Mendel then cross pollinate - He cut away the pollen bearing male part of the pea (so the self pollination will not be possible) and dusted its flower from another plant.

Therefore, he was able to get an offspring from two different parents.

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Image source: https://ka-perseus-images.s3.amazonaws.com/1d332b8f35d27ca033bd25ed9aa1f7470344cd80.png

Mendel’s results

He reported on seven of the numerous characteristics of the garden peas he studied.

He observed that each of the seven characteristics occurred in two contrasting forms:

The seeds were either round or wrinkled.

The seeds were yellow or green.

The flowers were red/pupple or white.

The pods were inflated or constricted.

The pods were green or yellow.

The flowers were axial or terminal.

The stems were long or short.

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Mendel’s results

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Monohybrid inheritance

He cross pollinated plants with contrasting forms of just one of these characteristic, he found that all the off-springs look alike and resemble just one of the two parents.

This he referred to as the first filial generation, represented with F1.

When these off-springs were crossed among themselves, it was observed that some of their off-springs showed one of the original contrasting traits and some showed the other.

This he referred to as the second filial generation, represented with F2.

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Mendel’s law of Dominance - hybrid offsprings will only inherit the dominant trait in the phenotype. The alleles that are suppressed at the first generation are called the recessive traits while the alleles that determine the trait are known as the dormant traits.

Parents Red flower (RR) X White flower (rr)

F1 generation All red flowers (100%) - Rr

F2 generation 705 red (approx. 75%) 224 white (approx. 25%)

¾ red ¼ white

F2 Phenotypic ratio 3 red : 1 white

F2 Genotypic ratio 1RR: 2Rr: 1rr

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Mendel’s cross of red flower with white flower peas

It was observed that one characteristic had taken dominance over the other, i.e. red colour had taken dominance over the white colour. Mendel called the trait (red flower) that appear in the F1 off-springs of such cross as dominant characters and the latent traits in the F1 as recessive characters.

Mendel law of segregation - each pea plant possesses two hereditary ‘factors’ for each character and when gametes are formed the two factors segregate and pass into separate gametes.

Image source: https://www.adefenceofthebible.com/wp-content/uploads/2017/11/Mendels-Sweet-Pea-Experiment.jpg 12

Allele (allelomorph) – an allele is a different form of a gene.

It is any one of two genes that could be found alternatively at a given site (locus) of a chromosome e.g.

R, r

• Therefore, a cell of a pea plant may contain two copies of the allele for red flowers (R/R), or two copies of the allele for white flowers (r/r) or one copy of the allele for red and one copy of the allele for white (R/r).

• Cells with two copies of the same allele (R/R or r/r) are refer to be homozygous for that trait.

• Those with one each of two different alleles (R/r) are called heterozygous.

Therefore, there are two possible phenotype – red and white and

there are three possible genotype – R/R, R/r, and r/r.

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The heterozygote (R/r) produces a red phenotype, because the red allele is dominant.

P R/R (red) X r/r (white)

F1 R/r (red) X R/r (red) [All red]

F2 R/R (red) R/r (red) r/R (red) r/r (white)

Genotypic ratio 1 R/R 2 R/r 1rr

Phenotypic ratio [3 red : 1 white]

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All the F1 progeny had red phenotypes, because all of them were heterozygous. The F2 progeny had three genotypes and two phenotypes. ¼ were homozygous dominant (red phenotype), 2/4 were heterozygous (red phenotype) and ¼ were homozygous recessive (white phenotype).

Some terms to consider:

Character – any structure, functional attribute, behavioral trait, or other characteristics of an organism.

Gene – the unit of inheritance; usually a portion of a chromosome that codes for a specific characteristic e.g. round seed.

Locus – a particular location/position on a chromosome

Alleles – alternative form of a gene, located at a specific position on a chromosome.

Dominant alleles – dominant form of the gene represented with capital letter e.g. ‘A’

Recessive alleles – traits that are latent in the F1 generation, often represented with small letter e.g. ‘a’

Homozygous – cell with two copies of the same alleles (A/A or a/a).

Heterozygous – cell with one each of two different alleles (A/a)

Genotype – possible genetic combination

Phenotype – possible appearance of an organism

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R/r red

R/r red

r/r white

r/R red

R/r red

R/R red

R r R r Gametes

F1

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F2

X

Possible genotypic combination in the F2 R/R

Genetics and probability

Probability is the likelihood that an event will occur.

The principle of probability has been used to predict possible genotypic combination in the F2.

Punnett square

- It is use to determine the possible genetic outcomes.

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R/R R/r R/r r/r

R r

R

r

Rr

Rr Male

Female

The test cross

• In a monohybrid (only one character is considered) cross involving dominance, the homozygous dominant progeny and the heterozygous progeny have the same phenotype and cannot be distinguished by physical observation.

For example, to determine if a particular purple flower pea plant has a genotype of P/P or P/p (both appears purple physically and has at least one P allele), it will be cross with a white pea (has only one genotype – p/p).

If it has P/P,

when crossed with p/p, all the progeny will be P/p (purple).

If it has P/p,

when crossed with p/p, ½ of the progeny will be heterozygous (P/p) and ½ homozygous recessive (white).

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Image source: https://image.slidesharecdn.com/chapter14-120213070450-phpapp02/95/chapter-14-mendel-16-728.jpg?cb=1329117962

• Partial/incomplete dominance

- When a heterozygous individual show effect of both alleles from the parents.

- The mixing of two alleles resulting in an offspring with a new phenotype.

For example, cross between homozygous red snapdragons and homozygous white snapdragons produce pink snapdragons. When the pink are cross among themselves they produce red, pink and white off-springs in a ratio of 1:2:1.

P R/R (red) X R’/R’ (white)

F1 R/R’ (pink) X R/R’ (pink)

F2 R/R (red) R/R’ (pink) R/R’ (pink) R’/R’ (white)

• Codominance

- When the alleles of both homozygous parents are completely expressed. i.e. the phenotype of both parents are simultaneously expressed in their offspring.

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Image source: http://images.slideplayer.com/19/5779435/slides/slide_12.jpg

• Dihybrid inheritance

Mendel later combine more than one character, he crossed peas with round yellow seeds with green wrinkled seeds

i.e. considering forms/shapes and colour characters.

All the progeny at F1 had round yellow seeds.

The F2 progeny showed four different phenotypes in ratio of about 9:3:3:1.

This implies that the genes for the two characters segregate independently during meiosis - independent assortment.

Mendel's law of independent assortment -

the alleles of two (or more) different genes segregates into gametes independently of one another.

This means that the allele a gamete receives for one gene does not influence the allele received for another gene.

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R/R G/G round yellow

R/R G/g round yellow

R/r G/G round yellow

R/r G/g round yellow

R/R g/G round yellow

R/R g/g round green

R/r g/G round yellow

R/r g/g round green

r/R G/G round yellow

r/R G/g round yellow

r/r G/G wrinkled yellow

r/r G/g wrinkled yellow

r/R g/G round yellow

r/R g/g/ round green

r/r g/G wrinkled yellow

r/r g/g wrinkled green

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R/r G/g round yellow

rG

Rg

RG

rg rG Rg RG

R/r G/g Round yellow

Phenotypic ratio 9:3:3:1 Punnett-square representation of an F1 dihybrid cross R – round seed; r – wrinkled seed; G – yellow seed; g – green seed