Genes are the basic unit of inheritance by which characteristics are passed on from one generation to another



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Heterozygous- When the alleles are different for a characteristic, then the organism is heterozygous for that characteristic.

  • Alleles- Different versions of the same gene.

  • Allele-One version of the pair of genes that appear at a particular location on a particular chromosome and control the same characteristics like eye color.



Punnett Square

  1. Identify the trait that is to be studied

  2. Assign a letter to represent the trait. Don’t use letters where the capital and the lowercase look similar, like S, O and W.

  3. Then draw the square. Since you are only investigating one trait and each trait has two genes, then the Punnett square should have four boxes.

  4. Place each parent's genes on the outside of each square

  5. B B

Bb

Bb

Bb

Bb

b

b



B- Dominant Brown eye color

b- recessive blue eye color



  • monohybrid cross is used to determine the dominance relationship between two alleles. The cross begins with the parental (P1 or P) generation. One parent is homozygous for one allele, and the other parent is homozygous for the other allele. The offspring make up the first filial (F1) generation.

  • Cross that looks at one trait

  • Originally, we know one parent is tall and its genotype is TT (remember, we're talking about what you look like in your genes, and you have two alleles, or gene options, for each trait), so we put those letters on one side of the square. The other parent is short and its genotype is tt, so we put those letters on the other side. Every baby plant gets an allele from each of them.

  • All the pea kids are all tall because, in peas, tall is dominant.

  • Genotype-heterozygous (half tall and half short)

  • Phenotype- Tall (short trait is hidden)

F1 Generation



T

T

t

Tt

Tt

t

Tt

Tt




  • Crossing the F1 generation tall plants (Tt) will give F2 generation plants consisting of 3 tall plants and 1 short plant.

  • The short trait which was missing in F1 generation appears in F2 generation. The trait did not disappear, it was hidden as it was a recessive trait.

  • Genotype (1:2:1)

  • Phenotype (3:1)

    • 3 tall, 1 short

F2 Generation



T

t

T

TT

Tt

t

Tt

tt






P

P

w

Pw

Pw

w

Pw

Pw

Pea plant with purple flowers (P) and another Pea plant with white flowers (w)

Purple color(P)-Dominant gene
White color(w)-recessive gene

F1 generation will have all purple flowers as purple is the dominant gene.
Genotype-heterozygous; Phenotype-Purple

F2 generation ¾ will have purple flowers and the ¼ will have white flowers (which skipped a generation).

Genotype (1:2:1)

1 Homozygous dominant


2 Heterozygous
1 Homozygous recessive


Phenotype (3:1)
3 purple, 1 white

F1 Generation




P

w

P

PP

Pw

w

Pw

ww



F2 Generation

Practice questions

In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?





A. None

B. 1/4

C. 1/2

D. 3/4

E. All

A genetic cross between two F1-hybrid pea plants having yellow seeds will yield what percent green-seeded plants in the F2 generation? Yellow seeds are dominant to green.



A. 0%

B. 25%

C. 50%

D. 75%

E. 100%

References

  • http://www.mensaforkids.org/teach/lesson-plans/peas-in-a-pod-genetics/

  • http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/01q.html

  • http://www.ducksters.com/science/biology/hereditary_patterns.php

  • https://www.youtube.com/watch?v=Mehz7tCxjSE

  • https://www.youtube.com/watch?v=cWt1RFnWNzk

  • https://www.youtube.com/watch?v=Lsj-Ij53CkA&t=181s

  • http://www.biology-pages.info/M/Mendel.html

Common Human Inheritable Traits and Pattern of Inheritance

  • These are some of the common dominant and recessive traits in humans that can be easily observed in people around you. You have two genes for them. One inherited from your father and one from your mother. The way that a physical trait is expressed, and hence the way you look is a result of either a combination of dominant genes, a dominant and a recessive gene or two recessive genes.

  • hairline shape-v shaped (widows peak) or straight

  • earlobe attachment-free ear lobes, attached ear lobes

  • tongue rolling

  • handedness

  • cleft chin, Dimples, freckles,

  • naturally curly hair

  • hand clasping

  • colorblindness

  1. Hairline

  • One example of a dominantly inherited trait is the presence of a widow’s peak(v shaped) at hairline. Straight hairline is recessive trait.

  • (W) is the dominant allele, and (w) represent the recessive allele.  An individual with a (WW) or (Ww) genotype will have a V-shaped peak at the hairline. Only ww individuals will have a straight hairline.




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