Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2.
There is no male to male transmission. It has the same inheritance patterns as autosomal recessive for human females. The son of a female carrier has a 50 percent chance of having the trait. Mothers of males who have the trait are either heterozygous carriers or homozygous and express the trait.
Remember: The father passes his X sex chromosome and all its genes to his daughters and his Y sex chromosome with its genes to his sons. Genes act in pairs, one from each parent for the females. For this mode of inheritance, males get their gene for the trait from their mother. Their children are an unaffected female, an unaffected male, and a heterozygous male. A Punnett square is a chart that allows you to easily determine the expected ratios of possible genotypes in the offspring of two parents.
The mating between two parents is called a cross. The Punnett square is named after its developer, British geneticist Reginald C. In this case, the gene is autosomal, and both parents are heterozygotes Aa for the gene.
Half the gametes produced by each parent will have the A allele and half will have the a allele. That's because the two alleles are on homologous chromosomes, which always separate and go to separate gametes during meiosis. According to Mendel's law of segregation, the alleles in the gametes from each parent are written down the side and across the top of the Punnett square.
Filling in the cells of the Punnett square gives the possible genotypes of their children. It also shows the most likely ratios of the genotypes, which in this case is 25 percent AA, 50 percent Aa, and 25 percent aa.
Mendel developed the law of segregation by following only a single characteristic, such as pod color, in his pea plants. In this example, both organisms are heterozygous for flower color Bb purple. Both plants produce gametes that contain either the B and b alleles. If the gametes from both parents contain the dominant alleles, the resulting plant will be homozygous dominant and have purple flowers. If the gametes from both parents contain the recessive alleles, the resulting plant will be homozygous recessive and have white flowers.
If the gamete from one parent contains the dominant allele and the gamete from the other parent contains the recessive allele, the resulting plant will be heterozygous and have purple flowers. For a monohybrid cross, we are only looking at a single gene. Therefore, the outside of the Punnett square will only have single letters single alleles.
For a dihybrid cross, pairs of alleles are used. This means the outside of the square will have pairs of letters. A Punnett square for a monohybrid cross is divided into four squares, whereas a Punnett square for a dihybrid cross is divided into 16 squares. How many boxes would a Punnett square need if three traits were examined?
The squares are filled in with the possible combinations of alleles formed when gametes combine, such as in a zygote. These types of crosses can be challenging to set up, and the square you create will be 4x4. This simple guide will walk you through the steps of solving a typical dihybrid cross common in genetics.
The method can also work for any cross that involves two traits. In the Parental P generation two homozygous plants are crossed: a plant that produces yellow round peas YYRR is crossed with a plant that produces green wrinkled peas yyrr. The Punnett square for this cross is not shown, but all of the offspring would be heterozygous have the YyRr genotype and produce yellow round peas.
To arrive at this:. Two heterozygous plants YyRr are crossed. What gametes do each of these plants produce? When gametes are produced, they can either have the dominant R or the recessive r. And, they can either have the dominant Y or the recessive y. Combine the R's and Ys of each parent to represent sperm and egg. Next the gametes are combined to form the offspring's genotypes written in the center of the Punnet square. The resulting offspring will have the following ratios:.
Single-gene autosomal traits include widow's peak and freckles, both of which are illustrated below. Widow's peak refers to a point in the hairline at the center of the forehead. Assume that the dominant and recessive alleles for the widow's peak gene are represented by W and w , respectively.
Because this is a dominant trait, people with the genotype WW and the genotype Ww will have a widow's peak, and only people with the genotype ww will not have the trait. Assume that the dominant and recessive alleles for freckles are represented by F and f , respectively.
Because it is a dominant trait, people with the genotype FF and the genotype Ff will have freckles, and only people with the genotype ff will not have the trait. What determines if a baby is a male or female? Recall that you have 23 pairs of chromosomes—and one of those pairs is the sex chromosomes.
Everyone normally has two sex chromosomes. Later, you will learn that due to nondisjunction, males and females may have one less or one extra X chromosome. Your sex chromosomes can be X or Y. The baby will have two X chromosomes, so it will be female. Notice that a mother can only pass on an X chromosome, so the sex of the baby is determined by the father. The father has a 50 percent chance of passing on the Y or X chromosome, so there is a 50 percent chance that a child will be male, and there is a 50 percent chance a child will be female.
This chance occurs for each baby. A couple's first five children could all be boys. The sixth child still has a chance of being a girl. A Punnett square can also be used to show how the X and Y chromosomes are passed from parents to their children.
This is illustrated in the Punnett square below. It may help you understand the inheritance pattern of sex-linked traits. One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot tell the difference between red and green. Males are much more likely to be colorblind than females because colorblindness is a sex-linked, recessive trait.
Because males have just one X chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. Because females have two X chromosomes, they have two alleles for any X-linked trait. Females can have one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision.
Since colorblindness is recessive, the dominant normal gene will mask the recessive colorblind gene. Females with one colorblindness allele and one normal allele are referred to as carriers.
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