Determining Sex of Offspring

Specification:

• Recall that the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male
• Describe the determination of the sex of offspring at fertilisation, using a genetic diagram

 

Definitions:

Autosomes/Autosomal Chromosomes: Chromosomes in a cell other than the sex chromosomes

Sex chromosomes:

• Human males have an X chromosome and a much shorter Y chromosome in each somatic cell. The X chromosome contains more genes than the Y chromosome.
• Females have a pair of X chromosomes.
• Human somatic cells have 22 pairs of autosomal chromosomes and a pair of sex chromosomes, X and Y.
   
  Determining Sex:
• During gamete production, the female gametes or eggs contain an X chromosome each.
• However, the male will produce two types of sperm-one containing an X chromosome, the other, the Y chromosome.
• When male and female gametes fuse during fertilisation, there is a 50% chance that the offspring could be male or female.
• A baby girl inherits one X chromosome from her mother and another from her father.
• A baby boy receives an X chromosome from his mother and a Y chromosome from his father.
  
   
  
   
  
     

Nutrition and Transport in Plants etc.

2.17 describe the process of photosynthesis and understand its importance in the conversion of light energy to chemical energy
2.18 recall the word equation and the balanced chemical symbol equation for photosynthesis 


Overall equation for photosynthesis:


*Ignore the second equation!! It is unbalanced, and is there just to illustrate the thought process as you convert from a word equation to a symbol equation.*

What conditions are essential for photosynthesis?

· sunlight

· carbon dioxide

· chlorophyll

· a suitable temperature

· water

Photosynthesis depends on enzyme reactions in the chloroplasts. Remember the effect of temperature on enzyme activity-enzymes have optimum temperatures that vary between different organisms. (Optimum temperature=this is the temperature at which the enzyme is most active, catalysing the largest number of reactions per second.) --Certain enzymes in plants have a high optimum temperature. E.g. the optimum temperature of the enzyme papain found in papaya is about 65°C.

How do guard cells control the size of stomata?

In sunlight:

·         The concentration of potassium ions (K⁺) increases in the guard cells

·         Chloroplasts in the guard cells photosynthesise. The light energy is converted into chemical energy used to pump potassium ions into the guard cells from neighbouring epidermal cells. This lowers the water potential in the guard cells.

·         Water from neighbouring epidermal cells enters guard cells by osmosis so that they swell and become turgid.

·         The guard cells have a thicker cellulose wall on one side of the cell (the side around the stomatal pore). Hence, the swollen guard cells become more curved and pull the stoma open.

At night:

·         The potassium ions accumulated in the guard cells during the day diffuse out of the guard cells.

·         This increases the water potential in the guard cells and water leaves them by osmosis.

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Heredity

This is elaborating a bit on Genes and Inheritance, with more info about the key words to further understanding.


Chromosome
This is a rod-like structure visible in the nucleus during cell division. It is made up of the molecule, deoxyribonucleic acid (DNA). Chromosomes carry the information for making new animal or plant bodies. This information is carried in DNA. Each chromosome may carry many genes along its length.

Gene
A gene is a unit of inheritance. It is a small segment of DNA in a chromosome where specific genetic (hereditary) information is stored. Each gene has a specific function. E.g. there is a gene which determines the height of the pea plant, another determines the colour of their flowers, and another for the shape of the seeds. The place on the chromosome where the gene is located is called the gene locus (plural: loci).

Allele
Each gene can have different forms. Different forms of the same gene are called alleles. E.g. the gene for the height of the pea plant has 2 alleles: short and tall. The gene for flower colour in pea plants also has 2 alleles: purple and white. The alleles occupy the same relative positions on a pair of homologous chromosomes. (i.e. chromosomes that have the same genes at the same loci but possibly different alleles.) Letters are usually used to represent alleles. E.g. the dominant allele for tallness in pea plants may be designated T and the recessive allele for dwarfness, t.

Homologous chromosomes
Homologous chromosomes:

• exist in pairs. One chromosome in the pair comes from the male parent and the other from the female parent.
• are similar in shape and size (with the exception of the sex chromosomes X and Y)
• have exactly the same order or sequence of gene loci. The alleles in these gene loci may not be the same. 

Phenotype

This refers to a trait which can be seen, e.g. the outward appearance or visible trait of an organism. Therefore, tallness in pea plants is a phenotype. 

Genotype

The genetic makeup of an organism. (Basically the combination of genes in an organism.)

• An organism is said to be homozygous for a trait if the 2 alleles controlling the trait are the same, e.g. TT (homozygous dominant) or tt (homozygous recessive) 
• An organism is heterozygous for a trait if the 2 alleles controlling the trait are different, e.g. Tt.

Dominant allele

This expresses itself and gives the same phenotype in both the homozygous and heterozygous conditions, e.g. tall plants have the TT or Tt genotype (phenotype: tall)

Recessive allele

Does not express itself in the heterozygous conditions. Only expressed in the homozygous condition (tt). Thus pea plants will only be dwarf if they have the 'tt' genotype whereas a tall pea plant may have either the TT or Tt genotype.