Outline how sperm are produced from diploid cells in the testis and how this production can be sustained over many decades of adult life.
- Germinal epithelium divide endlessly (by mitosis giving rise to spermatogonia);
1 mark - Spermatogonia are diploid/2n;
1 mark - Spermatogonia divide by mitosis / provide a continuous supply throughout adult life;
1 mark - (Some) spermatogonia enlarge forming primary spermatocytes;
1 mark - Primary spermatocytes undergo the first division of meiosis/meiosis I;
1 mark - Secondary spermatocytes produced are haploid/n;
1 mark - Secondary spermatocytes undergo the second division of meiosis (to produce spermatids);
1 mark - Spermatids develop tails
OR
Spermatids differentiate into spermatozoa / spermatids associate with Sertoli cells;1 mark
Marks can be awarded to an annotated diagram.
Do not accept sperm or spermatozoa as equivalent to spermatogonia or spermatocytes.
Testis cells are eukaryotic cells. Identify the structures seen under the electron microscope in testis cells that are not present in prokaryotic cells.
- Nucleus/nuclear membrane;
1 mark - Membrane bound organelles;
1 mark - Mitochondria;
1 mark - Rough ER/smooth ER/Golgi apparatus;
1 mark - Lysosomes / centrioles;
1 mark - Large/80S ribosomes / ribosomes attached to a membrane;
1 mark - Linear chromosomes / histones;
1 mark
Autosomal genes are located in chromosomes that are not sex chromosomes. The inheritance of autosomal genes is affected by whether the genes are linked or unlinked. Explain the two types of inheritance, using the example of parents that are heterozygous for two genes A and B.
- Unlinked genes are on different chromosomes / vice versa;
1 mark - Unlinked alleles migrate/segregate/are inherited independently (during meiosis) / vice versa;
1 mark - (In unlinked inheritance) there is an equal chance for all 4 options to occur / AB, Ab, aB, ab / vice versa;
1 mark - (Dihybrid crosses involving) linked genes do not produce Mendelian ratios;
1 mark - (Excluding recombinants) there is a 1:1 chance of inheriting the different options/AB or ab;
1 mark - In linked characteristics, alleles might not migrate together if there is crossing over/recombinants are formed;
1 mark - Crossing over occurs in prophase I of meiosis;
1 mark - When the sister chromatids migrate in meiosis II, the characteristics forming gametes are different/Ab, aB;
1 mark - Formation of recombinants causes changes in ratio/probability of inheritance/genetic variation;
1 mark - Correct named example of inheritance of linked/unlinked characteristics;
1 mark - Punnett/paired diagrams of both unlinked and linked characteristics;
1 mark - Genes which are linked but are far apart on the chromosome can display independent assortment;
1 mark
Mp a could be awarded from an annotated diagram.
Allow annotated diagram of inheritance / could be shown in a Punnett square.
Allow annotated diagram of linked inheritance for mp f.
For mp k, accept sex-linked examples involving two genes.
If the student interprets the question as sex-linked and autosomal inheritance, look for WTTE marks from the scheme.
