Genetic crosses, Heredity: pp 200 - 201 and pp 211 - 217

Be able to explain and diagram the following

:

  • Mendel determined the law of segregation through his work with flowering pea plants
  • Mendel coined the terms dominant and recessive traits
  • Genetic diagrams ( Punnett squares) show the probabilities of inheritance of a particular trait within a family
  • Pedigree charts diagram the family genetic history
  • Monohybrid crosses and test crosses
  • Sex-linked genes refer to those genes that are located on the X chromosome.
  • Blood types are an example of codominance and of multiple alleles


The link below shows the hereditary pattern of hemophilia in the family of Prince Alexis of Russia
http://www.anselm.edu/homepage/jpitocch/genbio/hemophilia.JPG

This link shows blood types
http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f21-7a_abo_blood_types_c.jpg
and blood types around the world

external image 2-Mendel-jardin.jpg
external image 2-Mendel-jardin.jpg


external image 5peaplants.jpg
external image 5peaplants.jpg

Mendel's principle of segregation states that during gamete formation the alleles

in each gene segregate and pass randomly into gametes

.A dominant trait shows as the phenotype whether the alleles are homozygous or heterozygous for that trait.
Some traits are recessive and, therefore, require both alleles to be recessive in order to show as the phenotype.

Genes and Alleles

Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.
Different forms of the same gene are called
**allele** [allele: One form of a gene. ]s (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
Alleles are
**dominant** [dominant: an allele that always expresses itself whether it is partnered by a recessive allele or by another like itself ] or **recessive** [recessive: describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene ]:
  • the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
  • the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair
For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes.





Genetic diagrams - how genes and alleles (genotype) produce a trait (phenotype):


Gregor Mendel (1822-1884) studied the inheritance of different characteristics in pea plants. He found that when he bred red-flowered plants
with white-flowered plants, all the offspring produced red flowers. If he bred these plants with each other, most of the offspring had red flowers,
but some had white. This was because the allele for red flowers is dominant, and the allele for white flowers is recessive.
Genetic diagrams help to show how this works.
In a genetic diagram, you show all of the possible alleles for a particular characteristic. There will be two alleles from one parent,
and two from the other parent, making four altogether.
You then draw lines to show all the possible ways that these alleles could be paired in the offspring. There will be four possible ways, but some or all of them could be repeated
.
Genetic diagram of FF x ff for flower colours
Genetic diagram of FF x ff for flower colours

Genetic diagram of FF x ff for flower colours

A genetic diagram showing the outcome of Mendel’s first cross. All the offspring have red flowers, even though they carry the recessive allele for white flowers

In genetic diagrams, the dominant allele is shown as a capital letter, while the recessive allele is shown as a lower-case letter.


Genetic diagram of Ff x Ff for flower colours
Genetic diagram of Ff x Ff for flower colours

Genetic diagram of Ff x Ff for flower colours

A genetic diagram showing the outcome of Mendel's second cross. Three-quarters of the offspring have red flowers and a quarter have white flowers

Below shows a genetic cross with

  • two homozygous parents RR and rr and their offsprinf (F1) which are phenotypically R but genotypically Rr.
  • When F1 (Rr) are crossed, their offspring (F2) are phenotypically R : r [3;1]
  • This is called a monohybrid cross; in this case, mono = one trait, hybrid = mixed or heterozygous genotype.












    external image image%3FbinaryId%3D774%26rendTypeId%3D4&usg=AFQjCNHprvzQEf1lHDJKFEjRYEqczz4BTA
    external image image%3FbinaryId%3D774%26rendTypeId%3D4&usg=AFQjCNHprvzQEf1lHDJKFEjRYEqczz4BTA

Test cross to determine the genotype of a dominant phenptype:


external image collflower2.jpg == ==

Some genes are sex-linked, that is, they are located on the X chromosome but not on the y chromosome.

external image x-linked-recessive-genetic-defects.jpg
external image x-linked-recessive-genetic-defects.jpg


Colorblindness is an example of a sex-linked gene:


external image pedigree_chart_copy.gif
external image pedigree_chart_copy.gif



BLOOD genetics

  • Blood types
  • Sickle cell
  • hemophilia

Blood types and genetics :

Some genes are codominant, that is they are both expressed phenotypically.

An example is found in blood alleles, in which the protein A and B are

Since there is a possibility of more than two alleles for this gene,

it is an example of multiple alleles.

The four main blood types: A, B, AB and O.



external image 9125.jpg
external image 9125.jpg


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Example of diseases caused by mutations (changes) in an allele -

Huntington’s disease

Huntington’s disease is an inherited disorder that affects the nervous system. It is caused by a dominant allele. This means it can be passed on by just one parent if they have the disorder.
The genetic diagram shows how this can happen.

The Huntington's allele is represented by H. The normal allele is h. Suppose 'Parent 1' has Huntingdon's, then they are Hh. 'Parent 2' does not - they are hh. The possible combinations of alleles in the children are Hh, Hh, hh and hh. So it is possible for the children to get the disease if only one parent has it
The Huntington's allele is represented by H. The normal allele is h. Suppose 'Parent 1' has Huntingdon's, then they are Hh. 'Parent 2' does not - they are hh. The possible combinations of alleles in the children are Hh, Hh, hh and hh. So it is possible for the children to get the disease if only one parent has it

The Huntington's allele is represented by H. The normal allele is h. Suppose 'Parent 1' has Huntingdon's, then they are Hh. 'Parent 2' does not - they are hh. The possible combinations of alleles in the children are Hh, Hh, hh and hh. So it is possible for the children to get the disease if only one parent has it

Genetic diagram to show the inheritance of Huntingdon's disease

Note that if you are doing the Foundation Tier paper you are expected to be able to interpret genetic diagrams. If you are doing the Higher Tier paper, you are expected to be able to draw genetic diagrams for any combination of dominant and recessive alleles.

Cystic fibrosis

Cystic fibrosis is an inherited disorder that affects the cell membranes, causing the production of thick and sticky mucus. It is caused by a recessive allele.
This means that it must be inherited from both parents. The genetic diagram shows how this can happen.

Genetic diagram to show the inheritance of cystic fibrosis

The cystic fibrosis allele is represented by f. The normal allele is F. Suppose both parents have alleles Ff. The possible combinations of alleles in the children are FF, Ff, Ff and ff. The alleles ff will cause the disease. So, although the parents do not have cystic fibrosis, they can produce children with the disease. The parents are called 'carriers' of the disease
The cystic fibrosis allele is represented by f. The normal allele is F. Suppose both parents have alleles Ff. The possible combinations of alleles in the children are FF, Ff, Ff and ff. The alleles ff will cause the disease. So, although the parents do not have cystic fibrosis, they can produce children with the disease. The parents are called 'carriers' of the disease

The cystic fibrosis allele is represented by f. The normal allele is F. Suppose both parents have alleles Ff. The possible combinations of alleles in the children are FF, Ff, Ff and ff. The alleles ff will cause the disease. So, although the parents do not have cystic fibrosis, they can produce children with the disease. The parents are called 'carriers' of the disease

Notice that the offspring with Ff are labelled 'carriers'. A carrier has one copy of the faulty allele, but does not have the disorder themselves. In this example above, both parents are carriers. They may not know they are, but there is a one in four chance of them producing a child who has cystic fibrosis. It is possible to screen embryos to see if they carry alleles for genetic disorders.

Genetic diagram to show the inheritance of cystic fibrosis when only one parent is a carrier

The cystic fibrosis allele is represented by f. The normal allele is F. Suppose one parent is FF and the other is a carrier, Ff. The possible combinations of alleles in the children are FF, FF, FF and Ff. So the parents cannot produce children with cystic fibrosis (ff). But they can produce children with alleles Ff, who will be carriers
The cystic fibrosis allele is represented by f. The normal allele is F. Suppose one parent is FF and the other is a carrier, Ff. The possible combinations of alleles in the children are FF, FF, FF and Ff. So the parents cannot produce children with cystic fibrosis (ff). But they can produce children with alleles Ff, who will be carriers

The cystic fibrosis allele is represented by f. The normal allele is F. Suppose one parent is FF and the other is a carrier, Ff. The possible combinations of alleles in the children are FF, FF, FF and Ff. So the parents cannot produce children with cystic fibrosis (ff). But they can produce children with alleles Ff, who will be carriers

In the example, one parent is a carrier, while the other does not carry the allele for cystic fibrosis. They cannot produce a child with the disorder, but they can produce children who are carriers.
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